An Uneven Chest is very common in people with Pectus Excavatum & luckily you can improve this through exercise.
A strict bodybuilding routine targeted towards developing the smaller pec will result in the development of this side & as a result will make your chest more even.
Any unilateral chest exercise is very beneficial for evening out a chest assymetry as it will enable you to isolate and put more load on your weaker and smaller chest. Examples of this include:
Unilateral dumbbell chest press
Cross body seated chest press.
One arm low dbell Fly
Unilateral resistance band fly
If you would like to see a video tutorial on how to do these exercises you can sign up for a trial of my membership site which has video tutorials of me doing each exercise.
Check that out here: https://www.fixpectus.com/offers/kJhAziGT
Before you get started training to even out your uneven chest it is important to note that: I do not recommend only training the smaller side, so please still train both sides.
Just train the smaller side to failure. Otherwise you will end up with even worse imbalances.
You should find that your bigger pec is stronger, so I recommend that you simply train your smaller pec first and then match the amount reps you did on your smaller pec with your bigger pec.
This will then result in less stimulus for the bigger pec considering it is already more developed which will result in overtime the smaller pec developing more due to it being subject to a higher training intensity and eventually it will get closer to the size of your bigger pec.
It is important to note that this can only improve the asymmetry not result in a completely even chest.
It is also important to note that no one is perfectly symmetrical whether it be their pecs or their face so this is not something to strive for.
The structure & placement of your muscles on your body cannot be altered, we can only alter the size of the muscle itself.
Watch this video below to learn more about Fixing Assymetrical Pectus Excavatum.