Looking at my anatomy book, it seems that one side of my orbicularis oris is slighly larger making one side of my jowl look bigger. One side is also more convex than the other making it seem larger.
Here are pictures to explain better (asymmetry is much more apparent in real life)
http://img6.imageshack.us/img6/3613/img0026ju.jpg

bigger side (more convex)
http://img528.imageshack.us/img528/9130/img0027f.jpg

smaller side (smaller and less convex)
http://img4.imageshack.us/img4/9029/img0029uh.jpg

