Although I was never a golden gloves child prodigy like the fellow in another thread, I did used to box and trained closely with a number of other boxers as well. Never really noticed the effect you mention, but I will point out that strength/flexibility imbalances between the illiopsoas and rectus abdominis muscles can bring about an effect like you describe -- a bulge (usually in the lower portion) in spite of a high level of definition. And considering that full situps are still a very popular exercise among boxers, and that situps are known to cause or exagerrate this imbalance for some people, my guess is that's what you're seeing.