It's way too easy to let a pet bunny get overweight, and without proper exercise, even the largest bunny cage is not going to be enough to keep your pet bunny fit.
Exercise is key for pet bunnies to thrive, learn how to be handled, and to allow them to escape boredom. 15 minutes of 'free time' daily in a supervised area (your bathroom, the mudroom, or even in a large dog kennel) can allow your pet bunny to get the motivation it needs to stay healthy, happy, and loved.
Also important for pet bunnies is a proper diet. Unlike the cartoon rabbits, if you give a pet bunny too many carrots or veggies, they will begin to gain weight from all the sugar and starch. Stick to their regular diet and maybe toss in a grape a day or a small leaf of lettuce, and your pet bunny will be just fine.