I think it's the case with many animals that large breeds don't live as long as smaller breeds. It's my understanding as well that the average lifespan of a giant rabbit is only 3-4 years, whilst for smaller rabbits it's about 7. It's also the case (or so I've been told) that house rabbits live an average of a couple of extra years, I imagine this is because it's easier to spot if your house bunny is ill, rather than outside where you wouldn't spend as much time with it / them.