@pastanaga - Yeah, I've definitely known people who will let their dogs get to a very lean state in order to make them more aggressive, which I think is despicable. Dogs and cats shouldn't be allowed to get overweight, of course, and they should definitely be kept in good condition, but going hungry is not fun and animals can suffer just as much as people can.
We tend to think of fat as the enemy, but every mammal needs a layer of fat to help regulate temperature and to keep as reserves in case of illness.
If I said someone was lean and mean, I would say it as a joke, as in they were in top condition and raring to go. An animal that hasn't been fed properly is most definitely not in that state.