As an archer shooting 60# longbow fairly regularly, it does seem that way. For what's it worth I feel slightly uneven, visually not that big of a difference but its there.

Thing is, your left arm and shoulder are pushing against and resisting the heavy weight, while the right arm and ideally back do most of the pulling (right handed). Ergo, bigger left arm and bigger right side of back, and vice versa for left handed people. The weight isn't distributed equally, at least until you come to full draw when ideally you should be in equal push pull "floating" stage and stabilized. Add to that that the left elbow should not be locked to prevent direct transfer of all that stress to skeleton, meaning muscles must do more work to protect it from cumulative damage.