They said in the movie that he gets stronger as he shrinks.
In the comics he's still fullsized human strength at ant size.
Which.... Isn't what I'm talking about.
They say it reduces the space between molecules. It doesn't reduce the number of molecules. It doesn't actually shrink the molecules. It doesn't shift his mass into another dimension or some other comic-book technobabble.
It simply reduces the space between molecules.
* *
Becomes:
* *
Same number of molecules. Same amount of mass.
Space between molecules weighs nothing (if it weighed something there'd be mass there, ergo, molecules.)
So if he has the same number of molecules and the same mass then he weighs the same, he's just a lot more dense. Now, whether or not a 200-lb man the size of an ant is *too* dense without causing ripples in the fabric of space is another question.
But at the very least he still weighs the same, which means when he stands on a man's shoulder, or the edge of a gun a man his holding, that's a 200-lb mass now in that place. The guy isn't going to just stand there not noticing it. A plastic model train isn't going to move one iota with a 200-lb mass on it. Infact, it'd probably be crushed.
I would have accepted any technobabble explanation that explained how he was able to shrink and still do all of this stuff. (Again, shifting things into another dimension or something so he has access to all of his mass it's just that right now the mass in reality only makes up a small space.)
But, no, they went with "closes the gap between molecules" as the explanation, which has consequences that can be examined with more scrutiny than any sci-fi explanation.
It also means nothing is physically happening to the size of his molecules, meaning they cannot be made smaller than molecules and even into the quantum realm.
How do you reduce the space between molecules beyond the size of a molecule?!