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 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 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.
Continuity mistake: When Ant-Man steps into the tub, he is next to a curtain. Shot swaps to a wide angle and the curtain is gone, only to reappear next to him a shot later.
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.
Chosen answer: Despite the explanation given in the movie for being able to reduce objects and people being "reducing the space between atoms", there are clear indications that there's more to it than that as that explanation wouldn't enable changes of mass (i.e. No way an ant could carry the full weight of a man even if he was reduced in size). Therefore there is some way of changing mass at the same time as size and the two aren't necessarily linked (Scott has low mass when riding ants but much higher when he's punching people). Based on this its apparent that when Scott swats Cross into the buzzer he had low mass. So the answer to the question is - it depends on the mass of the smaller person at the time of the collision.