If I'm not misremembering, that's the exact explanation they use for pym particles in the first Ant-Man movie. And yeah, mass and forces we see in that movie aren't consistent with that rule at all either. In his shrunken form he can go from riding flying ant bareback to punching a guy with the force of a regular size dude (not concentrated to the size of bb though, he should be punching holes in people). Furthermore, at the climax of the film he needs to "go subatomic" to squeeze between atoms, but he keeps shrinking out of control and ends up in a crazy abstract environment many times smaller than the smallest particles we know of...but don't pym particles just make atoms get closer to each other? Not shrink? It's a fun movie but I need to turn off my brain to watch it.
And then in later movies when he grows, shouldn't he also be the same mass then too? Think about it, he's as big as a parade balloon and a fraction of the weight, the dude would need to hold onto something just to not blow away.