My left pec has gotten slightly bigger than my right one. How do I even it out, and prevent it from happening again?
This is normal is you're left handed. The side you use more in your everyday movement is going to be slightly bigger. Another possibility is you aren't keeping the bar steady when you lift and are over compensating with the left side of your body. I see a lot of idiots at the gym lifting more weight than they should instead of using proper technique.