At the risk of replying to an obvious troll...
The hitboxes for the normal models are pretty good, the hitboxes for the current Circus models are a bit off in some cases (particularly the Husk, where his head occupies a complete void).
Also you have to consider server lag, and the fact that that the game runs on a rather old engine and doesn't have sophisticated physics / skeletal animation, so when a specimen walks over a beer can or bit of wood that is part of the scenery their whole model and hitbox instantly jumps up to that height.
"And why is that, why isn't pixel a pixel like in Crysis 2"
Crysis 2 isn't "pixel a pixel" as you seem to think, it uses exactly the same kind of hitboxes, it's just a much newer engine with much better animation / physics (and possibly netcode) that make things seem smoother. The difference being Crysis 2 is a brand new game with a budget of tens of millions that released at $60, whereas Killing Floor is a two year old low-budget game based on a seven year old engine, so it has somewhat less sophisticated graphics & physics.
Incidentally, Crysis 2's multiplayer was by all accounts flawed from the start and it's population is already diminishing. killing Floor however is still going strong two years later, despite all the 'terrible flaws' you keep complaining about. Interesting how spending tens of millions of dollars on marketing and engine development doesn't actually make a game any better...
