Similar problem awhile back.
1st- Are you left side dominant?
2nd- Do you have any other muscles with the same problem?
3rd- Is it genetic or a deformity that you've always had.
I noticed this profound difference in size after my girlfriend pointed it out in a photo. I always knew it was smaller but didn't know the severity.
I did this to fix it. This is no joke. For almost 4-5 years it took me. It started with the bench press. My right side was constantly failing before the left. I'm right handed by the way with a dominant left side. I don't understand that, but thats me.
Anyways. I overloaded the right side on the bench press to force it to grow stronger. Also did countless extra reps & exercises on the right side with dumbbell presses, flyes, pullovers, cables, 1-arm push ups...you name it, I did it. It took a long time, but it caught up. I didnt want to sacrafice progress on my left side. So I decided to work the left 100% and right weak pec 150%.
I don't know if this will work for you, but it worked for me. Alot of my right side muscles were smaller, but none more evident than my chest.
Some say to just lift heavy compounds and it will work itself out. I tried that and only made it worse. My advice would be to overwork and focus on the weak right side or it really will never catch up.
- Someone that had the same problem