Two things really stand out.
First is what's been mentioned before, that the model quality is very similar between the two games, and that it's really due to engine and animation improvements that it looks so much better this time.
The second is how, while the kf1 zeds were across the board taller, they FEEL bigger this time around. KF2 is a much faster, closer game, and when the zeds run right up to your face they feel like they're right there, and big, rather than in kf1 where you could be right up against a scrake and you'd be dying, but they didn't have the same sense of solid physical presence. Don't know what to attribute that to, but it's an impressive improvement.
Edit: Regarding looking better, another reason is most of the new models have significantly stronger aesthetics. While they look comparable in quality at the minute level, zooming out a little gives the new models a significant edge in appeal and feel. With the possible exception of the big guys, who don't quite look right next to their kf1 counterparts, but are perfectly fine on their own.
Edit 2: fleshpoounds really need to do more damage, though. I can't help but think of a big clown when I look at the KF2 fleshpound, and it's nothing wrong with the model. They just are too goofy and unable to deal damage for them to be a serious threat next to the actually dangerous kf1 FP. It makes the model look weak, just due to my experience playing against them.
Edit 3: And Scrakes should be skinnier. Their profile is too similar to the fp as it is. Not that there's much chance of mistaking one for the other, but just a little more to bring them closer to the kf1 model would be nice. Not taller, just less slablike. Maybe. As it is, ignoring glowy bits and animations, the sc looks as different from the fp as the slasher does from the clot. More differentiation in body type would work wonders.