One Arm Stronger Than The Other?

[#1Assassin]

dumbell curls are a very bad idea for boxing.

[Freelancer]

If it's the size of the left compared to your right that is bothering you I'd suggest lifting heavier and slower with the left. Bringing them both to an equal power and size will take a long time either way. If it's not that important to you just do as much as you can with both but keeping the right in shape while focusing a bit on the left might be the better plan of action.