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)
Continuity mistake: When Scott and Hope are kissing at the end, Hope has her hands on Scott. When the shot flips to the other side, her hands are down.
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.