The main question was a 29" BP, which weighs only 400 gms about right.

Firtsly I would check accuracy of your scale. Take a few tins of beans of known weight and weigh them. If your scale is about right... good.

If in fact Buddy does only weigh 400gms, next question is how old is he?. I would think he should be closer to 700 gms for that length, but them I have not kept many BP's. so maybe other input on this subject will be enlightening.