Like a slob because that eans you can fight.... Having big muscles doesnt mean you are some kinda uncoordinated dolt. A 220lbs Tyson wouldnt have looked out of place on a bodybuilding stage. I dont think theres a lightweight bodybuilder in the world as shredded as Bradley when he weighs in on the scales. Having Muscle DOES help. Simply lifting weights and not working skills OBVIOUSLY isnt going to work. But a skilled boxer who has a professional strength training regime with the help of a proper nutritionist, will be a better fighter than otherwise. Wlad Klitschko has plenty of skills yet he still looks like a freakin comic book character. Skilled, with a proper S+C coach and nutritionist> Skilled with an old timey, get up and a "run 10 miles everyday because thats what boxers always do, and dont do weights because they slow up down coach"
All things being equal it is relative to style. Ideal is relative, but striking based fighting in general lends itself to length and speed. Pure Boxing is probably long lean and wirey. Brawling it is probably stout, short, and more powerfully/durably built. Boxer Puncher probably hovers somewhere in between. Muscle bulk is probably the worst thing for a Boxer to build due to its metabolic cost. It is probably better for a boxer to carry 10lbs too much fat than 10lbs too much muscle.