Depending on what BennyC was suggesting, disproportionate musculature has serious implications for injury and mobility. For bodybuilders this isn't really much of an issue, simply because all those bunched up, bulky fibres aren't really meant to go through full ROM for things like sprinting, jumping or lifting. For models, this is even less of an issue as their objective is to have audience-appropriate musculature with low body fat. Look at Hugh Jackman: guy has no legs: so what? He's not actually going to do anything, other than look big in a vest. And then there are the discussions over film stars, models, etc. and their use of steroids to change size so quickly... For an athlete, being unbalanced is a serious problem because of the crazy biomechanics that can occur when over-developed, out-of-kilter muscles pull against non-existent synergists. Knee, shoulder and back problems are all quite happy to manifest at these points. Sprinting requires a lot of flexibility to go with those big leg muscles. And for those leg muscles to get any decent toque requires (sigh) a stable base... Which is why most athletic training involves sport-specific as well as complementary (assistance) exercises.