I think people are criticizing Schwartz where there really isn't a criticism to be made. If you see Backes get muscled off a puck, well, good defensive play; if you see Stewart get muscled off a puck, oh, he's lazy or soft; if you see Schwartz muscled off a puck, he needs to be demoted and hit the gym. The dude wins puck battles that he has no right winning if that were true.
The reality is that because of his quickness, skill, and size, he is able to squeeze around a guy or in a scrum where a larger player just cannot go. As a result, he's involved in puck battles that most other players cannot be a part of. As a result, he gets pinned and trapped, especially behind the opposing net, but I have yet to see such a situation hurt the Blues - it actually tends to buy them time because someone else wouldn't or couldn't have darted in and forced the opposition to defend like that.
Watch last night's game. He had a big hit to begin the game that really put a guy on his ass. He had multiple really good defensive plays that he only pulled off because of his balance and strength - one play in particular behind the Blues' net.
Does he get shoved around at times because of his size? Sure. But that's also partly because of his inexperience. Just don't assume that every time he gets shoved around, it's because of his size. The NHL is filled with good, strong players who will do that against nearly everyone. And more often than not, when Schwartz does get shoved around, it's because he's paying the price to make a play, not simply being manhandled because he can't contend.