Alright so here's what my explanation for how Pym particles work would be:
Fundamental particles - quarks and leptons - have zero size. The sizes of protons, neutrons, nuclei, atoms, molecules, and so on, depend on the properties of the strong nuclear force (at very small scales) and the electromagnetic force (at larger scales). Those properties depend on certain physical constants. All of that is accurate to real-world physics
If the Pym field is in its ground state, then everything's fine, but if the Pym field is in an excited state, those constants stop being so constant and the distances between those fundamental point particles can get a lot smaller or a lot bigger
Pym particles are excitations of the Pym field
An excited Pym field has a weird effect on mass: basically, if a bound state of two or more fundamental particles interacts with a Pym particle and you measure that bound state's mass, the measurement will depend on whether the whole thing is moving relative to you or not. If it is, the mass is the same as if the Pym particle wasn't there. If it isn't, the mass is either increased or decreased. Something like that would get complicated when we're talking about many-particle systems like, say, a person, but it would be the excuse for why Ant-Man can stand on someone's shoulder without sinking through but his punches are "like a bullet."
(Similar vaguely scientific explanations would also apply to other comic book characters with shrinking powers)
