The subject of intentional breeding or meat rabbits is prohibited. The answers provided on this board are for general guideline purposes only. The information is not intended to diagnose or treat your pet. It is your responsibility to assess the information being given and seek professional advice/second opinion from your veterinarian and/or qualified behaviorist.
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when you said "it" said a rex should be 4.5 pounds, what or who is "it" Though there is an average for different breeds, there are bigger and smaller sizes within a breed. So the only way to see if your bunny is overweight is by feel and/or by a vet.
Here is a good tidbid from rabbitnetwork.org to help people tell if their bunny is fat."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!"