Pectoral imbalance in the inner pecs, one of my inner pecs looks full while the other looks like there is nothing there, which exercises can balance it out?
Not really an answer to your question but the left side of the chest can be a bit bigger since your heart is located underneath.
If I where you I would start training your chest muscles sepertly since you must have some kind of symitry since only one side of the chest Is growing. Meaning that one side is taking load of from the other and working more while the other less.
The dumbell benchpress is deffiently a must for you.
Flies with cables are also a great chest exercise that let's your chest muscles work independently from each other.
A great YouTube channel with lots of tips and training advise I recommend is athlean x