You would think that spaying something big would be a lot easier than spaying something small. With something big, you can visualize everything more easily, right? It turns out to be the exact opposite, actually. The smallest thing I've spayed is a two pound kitten, and oh boy is that uterus easy to find and manage. The biggest thing I have spayed is a hundred something pound Great Dane, and wow was that uterus deep in a deep abdomen and so covered in fat that it was hard to see where it stopped and the ovaries started. Big things bleed a lot; big things have ligaments that are really hard to break down (as in, you will get out of breath); big things have all kinds of extra fat and tissue and such that get in the way. Little kittens (and cats and puppies) have pristine little uteruses that pop right out at you.
This is something that vet schools don't make clear enough, in my opinion -- at least, not the one I went to and not the one that I work at now. I was supervising some beginning surgeons recently. Day Two was amusing: everyone who had spayed a cat the day before was spaying a dog, and they were all complaining about how hard it was. Everyone who had spayed a dog the day before was spaying a cat, and they were saying things like "I'm so much better at this than I thought I was!" No, you're just spaying a much smaller animal.
Given all of that, it bewilders me that vet schools seem to tend to start students off on dog spays. Why not cat spays? They are so much easier. Why start a beginning student on the hardest possible case? This is yet another brick in the wall that stands between me and understanding how veterinary education makes any sense at all.