Trivia: When Scott goes sub-atomic during the climax, pay close attention. For a very brief instant, there are a few flash-frames of a human figure, and a human silhouette can briefly be seen reflected on Scott's mask. It's very easy to miss the first time you watch the movie. These are indeed meant to be hints that Hank's wife Janet is still alive in the quantum realm.
Trivia: One of Cross' henchmen has a ten rings tattoo, a reference to the Mandarin from Iron Man 3.
Trivia: The yellowjacket's plasma cannons sound exactly like the AT-AT from The Empire Strikes Back.
Trivia: Garrett Morris makes a cameo as the man in the car when Scott hits its roof. Morris was the first person to portray Ant-Man in a 1979 sketch for Saturday Night Live called Superhero Party.
Trivia: Several costumes were made, all identical. The ones worn by Paul Rudd when he is standing talking and walking were made of many different metal parts and leather. The stunt suits were all made of foam rubber so all metal belts, straps were one piece instead of metal interlocking pieces. 17 different helmets were made of metal and again foam rubber, also ones that were open at the front so we can see the face, some fitted with yellow lenses and ones fitted without. Once Paul Rudd was wearing the helmet the chin strap was added and screwed into position. The stunt helmet was foam rubber and simply worn on his head - it was pull on / pull off in case of injury.
Trivia: Director Peyton Reed was up for consideration as one of the directors of Guardians of the Galaxy, but that project was ultimately given to James Gunn. However, Reed was not forgotten - after Edgar Wright had exited the Ant-Man project, Peyton Reed was then given the opportunity to direct Ant-Man.
Trivia: While denying the rumored existence of an Ant-Man, Darren Cross uses the phrase "Tales to Astonish." Ant-Man's first appearance in a Marvel comic form was under the title Tales to Astonish #27 (January 1962).
Trivia: Yellowjacket was another identity used by Hank Pym in the comics.
Trivia: In the final scene, Luis is talking with Scott about his friend's date who met up with The Falcon one time. Luis stated that The Falcon was asking that he is looking for a guy and the woman talks about the guys she knows. One of the guys Luis said she mentioned is a guy who climbs walls. This is a clear reference to Spider-Man, who is set to appear in the Marvel Cinematic Universe very soon.
Trivia: There is at the very end a post credits scene where Captain America and Falcon are talking with the Winter Soldier.
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.