Ok I'll try to give some rationales/causes for some of the errors in these.
Well one reason for them having different proportions is taht in roguelikes, most of the time, each monster fits inside a tile, regardless of the monster's size. In NetHack a five-headed boss dragon is fit inside a 32x32 square, just the same as an orc or a fox. Maybe you know that already, but berhaps you don't because roguelikes are quite the niche games, having such a masochistic appeal and whatnot, like most of the time NOT EVEN HAVING GRAPHICS. So, very counter-intuitively, a smaller character ends up looking bigger because monsters aren't in scale.
About the lightsource, my you're right, the highlights are probably incoherent all over the place.
About the different styles... Well, the dwarves were the first tiles I did without any kind of reference, the game had some placeholder tiles lifted from I don't know where, and I just made up these dwarves, and that's how they came up. I tried to give them an air of civilized people, so that's why they're brighter than the other ones...
And about the zombies, well, I don't like zombies =/ that's probably why they turned up worse. The center one is probably the crappiest. Unreadable funky torso.