My dwarf lop, Harvey, was 2.75 kg when he went to the vets, and she said he was really overweight because that weight was big for a dwarf and he had a bit of excess fat around his ribs, and she wanted me to get him down to 2.2 kg (that's a lot of weight to lose!!!). I got him down to about 2.5 and he was looking very slender and lethargic, so I then maintained him at about 2.6. I then spoke to another vet and she looked him over and felt him and said that excess body-fat wise he only had a tiny bit excess (they should have none she said, so a tiny bit isn't too bad, and I wasn't worried as this was just as winter was coming on and he's an outdoors bunny). She also said that his weight would be more than a normal dwarf as he's almost the size of a non-dwarf rabbit just in his build (although his ears are the proportion of a dwarf, so he is still a dwarf she said...), so I'm a bit annoyed the other vet was angry at me for his weight, he will not be going down to 2.2!
Basically if the vet doesn't think they're overweight they probably aren't as vets are likely to pick up on this issue as it can be a big problem with rabbits, and although weight does depend on the breed, like mine you might have an abnormally large (or small) rabbit for the breed, so just check they don't feel as if they have too much fat covering their ribs and spine, but the vet probably would have mentioned if they were overweight!