Factual error: Twice in the film it is made clear that the Pym particle works by reducing the space between atoms in order to shrink an object, and by increasing it to enlarge them. This means that the object will weigh the same, whether shrunk or enlarged - it cannot be otherwise. A 90kg man the size of an ant would punch a hole through any surface upon which he stood (and couldn't ride ants), Doctor Pym has been walking about with a 60 tonne tank in his pocket, Darren Cross lifts a full grown sheep between finger and thumb, and the supersized Thomas The Tank Engine would be far too light to crush the police car (in fact it would float harmlessly away as it would probably weigh less than the air it displaced).
Factual error: In order to enter the Pym Tech building, they drop the pressure in the water main and Scott floats in on a raft of fire ants. Getting into it the supply line would mean they have to enter from the municipal main, which would be under full pressure. Even with reduced pressure, the interior pipe would still be full of water. Additionally, Scott would not be able to enter through a faucet unless someone had left it running (the valve would be closed).
Continuity mistake: Cross is holding a normal striker fired (no external hammer) Glock handgun against the head of Ant-Man. In order to save his life, Hope orders the ants to block the firing mechanism of the gun, and they block a suddenly-visible hammer on the Glock. (01:26:00 - 01:27:00)
Factual error: In the water pipe scene, water pipes don't have printing on the inside of them. Bolt flanges are not inside pipes, and welds are not that close together, nor are the welds inside the pipe. Also how did he get to the faucet without someone turning on the water to that particular sink?
Continuity mistake: When Cross is on the model rail track, you see Thomas the tank engine fall off the track when he hits Cross. But in the next scene Cross throws him from the track. (01:40:00)
Factual error: If the Pym Particle works by reducing the space between atoms to make an object smaller, the atoms themselves remain at normal size but are more closely packed together. It would, therefore, be impossible to "go sub-atomic" since the object obviously couldn't shrink and be smaller than the size of the atoms it was composed of.
Other mistake: Scott Lang is swept out of the tub after having shrunk for the first time. When he lands on the ceramic bathroom floor, falling roughly two feet, there is an audible glass breaking sound and it appears he has broken the flooring with his fall. He then falls through a plaster ceiling to free fall down to a turn table, landing on a plastic vinyl record played by a DJ. The album does not break despite him having fallen from three times the height and landing on a weaker material. Moments later, he is thrown through a second storey window and lands on a car in the street and his weight is now once again sufficient to form a small dent in the hood. (00:31:15)
Continuity mistake: When Cross is seen inside Hank's home, he is wearing black gloves, a few moments later, in the same scene you can see that he is not wearing them anymore.
Continuity mistake: When they are going over the plan for the last time at Pym's house a close up of Luis shows his top button undone, then the next close up it's done up with no time to do it.
Factual error: Ant-Man is riding carpenter ants after a helicopter in flight. The ants are coming at the helicopter from under it as it rises off the helipad. This wouldn't be possible due to the down force of air from the rotors. (01:30:00)
Factual error: Near the end, Scott Lang inadvertently shrinks to subatomic size, drifting through a kaleidoscopic quantum universe, and we see him reacting to the visual effects. However, at subatomic size, Scott shouldn't be able to "see" anything, because the rods and cones in the human retina can only "see" in a narrow band of light frequencies, and Scott is far, far smaller than the frequency of visible light waves. Additionally, Scott couldn't possibly "hear" his daughter Cassie crying "Where are you, Daddy?" Human hearing is based completely on air vibrations at certain frequencies, and Scott is millions of times smaller than sonic frequencies or even the nearest air molecule.
Continuity mistake: After getting fired from Baskin-Robbins, Scott goes "home" to the Milgrom Hotel. As he approaches, there is a bicycle chained to a parking meter in front of the hotel; it is also visible in the close shot through the gate from inside. In the long inside shot from down the hall as Scott actually enters, the bike is gone.
Continuity mistake: Towards the end while Scott, Paxton and Cassie are eating, the steak on Cassie's plate moves between shots.
Factual error: When going sub-atomic, Scott passes into an atom. The image shown is of the Rutherford model of the atom with electrons orbiting the nucleus. This model is incorrect and has been replaced with the more-accurate Bohr model of the atom.
Factual error: In the scene when The Wasp (Hank's wife) goes subatomic to deactivate a missile, you see the missile flying horizontally in the clouds. That is not possible. First, that missile does not have wings so it simply could not fly horizontally like an airplane. Second, Hank states it's a ICBM (Intercontinental Ballistic Missile). ICBMs have a circular trajectory with apogee at 1200km of altitude (750mi), WAY above the clouds.
Continuity mistake: During the train fight at the end Thomas the Tank Engine is knocked completely off the track and lands on the table, but in the next shot it's lying on the track, where it is picked up and thrown at Ant-Man. Additionally, Yellowjacket is not visible when the train collides with him.
Continuity mistake: When Scott uses the suit for the first time, he is on a tube with a small red towel on it which disappears between shots. Not only this - the amount of food on the plate reduces and then increases during the scene.
Continuity mistake: When Scott escapes from jail with the help of the ants, as he's flying through the trolley car he lands on a newspaper. From the position of the man's thumb, the picture is just a small part of the left side (his thumb lays over about half the width of the picture). When it cuts to the wide shot, the picture now covers more than half the page (width-wise).
Continuity mistake: When Scott wakes up after falling off Anthony, we can see the floor around the bed and there are no ants there. He then looks down and sees the bed is surrounded by ants.
Factual error: As they are heading back to the city, the scene switches back and forth from Luis to Scott. There is nothing but shrubbery visible through the window to Scott's right, but suddenly, there are two cuts in which they appear to be very close to an unidentified bridge. The closing shot on the scene shows the bridge to still be way off in the distance. The bridge would not have been off to their right if they were still approaching from the north.
Chosen answer: The suit was built human-size and then shrunk using Pym particles. The same technology can also be used to grow objects. Cross was shown to have a hand-held device earlier in the film capable of shrinking, so he either used this device, or activated the suit's built-in growing/shrinking technology.