I have little boobies.
They were HUGE when I was nursing. I wasn't used to it.
I think it's pretty normal for a child to prefer one over the other, much like I am more comfortable sleeping on my right side.
I had two kids 10 years apart. I guess I'm pretty lucky. At 50, I have nice, small, perky little breasts, but I believe it's mostly due to wearing good, supporting bras, pregnant or not.
I would assume that your one breast is bigger because that's the one the child prefers and therefore produces more milk. You really should try switching it up as much as possible.
It's just my opinion, but nursing at 2 years old is a comfort thing. Your child is old enough to say to him, "Honey, we're nursing on the other side this time to help mommy out".