My understanding is that it takes a full year for the bone structure to stop growing and changing. Doesn't necessarily mean they will get larger during the whole year, but the structure can change - for example, facial structure will continue to change.
Regarding how to tell if your bunny is fit: This is from a great article I copied and pasted.
"How do you know if your bunny is fat? Many house rabbits are, and they are not at all embarrassed by it! Rabbits store fat inside their abdomen, which makes just eyeballing them for love handles ineffective. Run your hands along your rabbit's side, feeling the rib cage. When you reach the end of the ribs, your should be able to feel a slight inward slope, the waist. Fat bunnies will balloon out instead. Remember that the ribs are palpable in even quite obese rabbits, so don't let your bunny fool you into more snacks!"