In Ant-Man, it's (briefly) explained that the Pym particle reduces the distance between molecules, allowing a person to shrink while retaining his mass and strength.
Much of the movie is consistent with this explanation. For example:
- While ant-sized, Ant-Man falls from the edge of bathtub onto a tile floor, cracking the tile as though with the weight of a full sized human.
- He falls from a rooftop onto a car, denting the roof.
- He punches full sized humans, and they respond as though hit by a real punch.
- Another character, while tiny, is hit by a speeding toy train. The train is knocked aside as though it hit a full sized human.
But then there are things that appear totally inconsistent with "mass stays the same":
- Ant-Man is constantly running and climbing around on surfaces (air vents, ant tunnels, etc.) that couldn't possibly support the weight of a 200 lb man on a footprint the size of an ant.
- The toy train, when expanded to the size of a real train, is heavy enough to rip through a wall and crush a police car.
Hank Pym carries a shrunken tank in his pocket on a keychain. It obviously doesn't weigh as much as a tank, but it does when expanded to full size.
- Another character, while tiny, gets swatted with a (normal sized) table tennis paddle, and goes flying like an insect.
Obviously the out-of-universe explanation is that mass behaves in whatever way is most convenient at the moment because it's just for fun.
But I'm curious: is there any in-universe explanation, either hinted at in the film itself or in the comic books, as to why mass is conserved sometimes but not always?