You're mixing up real with realistic/realism. A design doesn't have to be 100% real to be realistic or believable. Its the difference between a powered exoskeleton outfitted with ceramic armor plating and Gurren Lagann. One of those things is believable, the other is just far out there.
So far, the weapon designs have always looked like real weapons, or at least had the shape of realistic weapons. But weight, from what we've seen, is the same in TES as in real life. Warhammers, as show in Morrowind and Oblivion, are just totally ridiculous, whether you look at them from a real-world view or an in-world view. They're straight up stupid, and I really don't understand why it seems like folks are defending them. They're terrible looking weapons.
Part of the problem is, I think, that for anyone who isn't a medieval weapons buff (who knows what one looks like), a real warhammer? Kind of silly looking and unimpressive. Since I doubt they wanted that reaction from their audience, they went with something more bulky & sledgehammer-like. (Yeah, they might have gone overboard a bit, but......)
So, what looks "realistic"/believable to the random viewer, isn't realistic to a scholar of old weapons.
...certainly, nothing in the TES games is remotely as "out there" as the weapons from WoW, or Final Fantasy, or Monster Hunter.
(Heh. Looking at the pictures of the default weapons on the wiki, I'm reminded of the larger issue with Oblivion weapons... the weight. Those huge warhammers don't seem so unwieldy when you think about the fact that your average fighter can easily swing around a 20-40lb
one-handed sword. I really hope they come up with a better way to restrict/"balance" the weapons in Skyrim than to give them such high weights.)