Ok, maybe I understand it wrong but I thought they said your mass stays the same but your atoms change positions making you smaller. So you weigh the same but are just tinier? If that is true, wouldn't that be a LOT of weight on Michael Pena's shoulder?
Also, wouldn't the increased ant and increased train car be VERY light? Would it really have been the weight of a real train?
Here's my take on rationalizing the science of this movie:
They specifically say they are reducing the distance between atoms
Maybe that's what they "think" is happening. I believe in the comics they suggest that matter is instead being shunted and pulled from another dimension.
They specifically say that shrinking increases strength and density
This is the biggest flaw. Clearly if he weighed the same as an average sized man he wouldn't be able to ride on the backs of ants and be sucked up by a Hoover vacuum. If the prior assumption is true, that would at least alleviate this issue however it wouldn't explain the "force" behind his punches.
Other characters can hear shrunken Ant-Man speak
I really, really wanted a scene showing how the helmet includes a special amplifier/voice changer to fix his tiny voice to be deep and loud but we never saw it. So, I'll just assume that's how it works.
Shrinking only effects specific objects
The suit and those discs simply need to touch an object to shrink and grow them. So how to they know where to stop? For example, when the train toy is enlarged, why isn't the window? Well, I suppose you could assume that Pym particles only react to the outer most atomic structures that are connected by their electromagnetic force and stop at the point where other structures electromagnetic forces oppose them. This is what keeps your hand from moving through your desk. The atoms aren't actually touching. They are being repelled like magnets against the atoms of the desk.
Ant-Man can go sub-atomic
Yeah, so even if you could shunt matter into another dimension, there becomes a point where you would be reduced to a single atom and still be larger than sub-atomic. So, hmm. Yeah, I haven't got a clue on this one aside from "MAGIC!"