Why?

This. As simple.

I understand being a newbie you would wonder about these, not that I blame u for asking it here but I guess the answers are readily available in the net. You get to learn a lot more stuff besides what you asked. So go spend a few hours reading on stuff and then come ask questions. Cheers

soooo, you gave up unilateral movements because you had bad form from the start, therefore causes your left arm to be smaller?

of course it wont catch up with your right arm, your right is compensating for your left. i can gurantee you that.

fix your db curl form and start doing it again