You're right Dry. This is also a quite logical answer. Here's the reasoning. More than likely you train both biceps during the same time, with the same exercises, and with the same weight. Therein it goes without saying that both muscles should develop in a similar manner. This applies to almost all muscles. When you bench you use both pecs with the same weight, exercise tempo, etc. When you do tricep dips you again use both triceps against your bodyweight + x, same tempo, time, etc. This applies to all muscles in almost all cases.
I don't believe that there is a weight bearing symmetry between muscles though. You may not train your pecs the same way you train your gluteus. Your pecs may have been trained in such a way to develop more fast twitch muscle fibres, whereas your gluteus may have developed slow twitch fibers, and vice-versa. Some people are born with a natural tendency to be able to utilize their gluteus and potentially develop it multiple degrees above what they are capable of doing with their other muscles, for instance.
Body symmetry (the one I believe you're thinking about) is actually just that. Does the right side of your body look symmetrical to your left side? There have been various tests on this actually in the context of the female orgasm. Sex-psychology studies have shown that females were more likely to experience orgasms with partners who were physically symmetrical, as this apparently represented a healthy body and a desirable genes.
We know that people can develop muscular dissymmetry from weight lifting, and while they may be healthy, the dissymmetry would still (should still, actually) affect the female orgasm in a similar way.
But now I'm just going off on tangents.
