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.
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.