Scott Lang: Pick on someone your own size.
Scott Lang: I didn't steal anything! I was returning something I stole.
Darren Cross: You tried to hide your suit from me, now, it's gonna blow up in your face, and destroy everyone you care about.
Scott Lang: Sorry I'm late, I was saving the world. You know how it is.
Hank Pym: Scott, I need you to be the Ant-Man.
Scott Lang: One question... Is it too late to change the name?
Scott Lang: My days of breaking into places and stealing shit are over! What do you need me to do?
Hank Pym: I want you to break in a place and steal some shit.
Scott Lang: Makes sense.
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.