Lazy rabbits are usually smaller because they don't have the ambition to fight their littermates for their share of the feed. Laziness in a rabbit is a personality that must be watched. I have a couple of rabbits that are lazy by nature. I don't worry about them because they have acted like that since they was kits. But if an active rabbit starts to act lazy I start to worry.
Hot weather can cause a rabbit to act lazy.
But laziness in an active rabbit can be the first signs of sickness. Laziness that is the result of overfeeding can be a major problem to deal with. I think one of the prettiest sights in rabbit raising is to see 3 or 4 kits sleeping on their mothers back. Once in a while, a kit will even decide to sit on the mother's head. A lot of times the kits will have a contest to see who gets to sit on top of the mother.