It varies! :lol: It's hard to tell without seeing the rabbit in person. You should be able to feel the spine when you run your hand over it but it shouldn't be protruding, same with the legs. A buns tum size is not a good indication of weight.
From looking at your signature, your bun looks around Old English size, and they should be around 3 kilos. Obviously this varies from rabbit to rabbit, and if your bun is a cross that changes things too. If you could post a bigger pic of your boy/girl someone may be able to give you advice breed wise. :wave: