Factual error: Twice in the film it is made clear that the Pym particle works by reducing the space between atoms in order to shrink an object, and by increasing it to enlarge them. This means that the object will weigh the same, whether shrunk or enlarged - it cannot be otherwise. A 90kg man the size of an ant would punch a hole through any surface upon which he stood (and couldn't ride ants), Doctor Pym has been walking about with a 60 tonne tank in his pocket, Darren Cross lifts a full grown sheep between finger and thumb, and the supersized Thomas The Tank Engine would be far too light to crush the police car (in fact it would float harmlessly away as it would probably weigh less than the air it displaced).
Factual error: In order to enter the Pym Tech building, they drop the pressure in the water main and Scott floats in on a raft of fire ants. Getting into it the supply line would mean they have to enter from the municipal main, which would be under full pressure. Even with reduced pressure, the interior pipe would still be full of water. Additionally, Scott would not be able to enter through a faucet unless someone had left it running (the valve would be closed).
Factual error: In the water pipe scene, water pipes don't have printing on the inside of them. Bolt flanges are not inside pipes, and welds are not that close together, nor are the welds inside the pipe. Also how did he get to the faucet without someone turning on the water to that particular sink?
Factual error: If the Pym Particle works by reducing the space between atoms to make an object smaller, the atoms themselves remain at normal size but are more closely packed together. It would, therefore, be impossible to "go sub-atomic" since the object obviously couldn't shrink and be smaller than the size of the atoms it was composed of.
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)
Factual error: Near the end, Scott Lang inadvertently shrinks to subatomic size, drifting through a kaleidoscopic quantum universe, and we see him reacting to the visual effects. However, at subatomic size, Scott shouldn't be able to "see" anything, because the rods and cones in the human retina can only "see" in a narrow band of light frequencies, and Scott is far, far smaller than the frequency of visible light waves. Additionally, Scott couldn't possibly "hear" his daughter Cassie crying "Where are you, Daddy?" Human hearing is based completely on air vibrations at certain frequencies, and Scott is millions of times smaller than sonic frequencies or even the nearest air molecule.
Factual error: When going sub-atomic, Scott passes into an atom. The image shown is of the Rutherford model of the atom with electrons orbiting the nucleus. This model is incorrect and has been replaced with the more-accurate Bohr model of the atom.
Factual error: In the scene when The Wasp (Hank's wife) goes subatomic to deactivate a missile, you see the missile flying horizontally in the clouds. That is not possible. First, that missile does not have wings so it simply could not fly horizontally like an airplane. Second, Hank states it's a ICBM (Intercontinental Ballistic Missile). ICBMs have a circular trajectory with apogee at 1200km of altitude (750mi), WAY above the clouds.
Factual error: As they are heading back to the city, the scene switches back and forth from Luis to Scott. There is nothing but shrubbery visible through the window to Scott's right, but suddenly, there are two cuts in which they appear to be very close to an unidentified bridge. The closing shot on the scene shows the bridge to still be way off in the distance. The bridge would not have been off to their right if they were still approaching from the north.
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.