OK, here's a theory as to how this muscle imbalance of mine could have come about:
When I started with stronglifts the weights I was pushing were quite puny for bench press, I didn't bother to use clips to keep the weight plates on the barbell (plus with only one plate on each side clips don't seem that necessary). Ultimately though, if one side is stronger than the other the plates can start to slide off.
Being right handed my right side is stronger, so the left plate would tend to start sliding. I then started to make a concerted effort to keep the bar level. These days I am pretty good at keep the bar level but quite possibly the left side is working harder to achieve this, hence the left pec has grown more. I am assuming that even though now that my left pec is bigger, it doesn't necessarily mean that it is stronger than the right. Am I way off the mark here or does this all sound plausible?
edit: Unless it's some messed up combination of left pec and right arm doing all the work in which case right pec is not getting full stimulated

FML
There is a myriad of reasons that could contribute, what you explained is totally possible because at the end of the day the bottom line is that one Pec compensated more than the other one, how this happened is really irrelevant at this point. What you need to do from here on out is well basically focus more on form/movement ensuring you concentrate more on your "smaller" pec.
Dumbbells does a perfect job for this. Keep in mind most cases "uneven" shaped chest is usually a case where you are lifting slightly more than you can handle and then the chest overcompensated. Exactly how the chest compensated is dependent on on many factors example using more shoulder power from right forcing left chest, or just plain pushing hard left contracting the muscle longer, or the physical structure of your backbone forcing your left side etc.
It's not too a big deal as you can correct this by using dumbbells. I believe Bench will make you strong and dumbells will make you pretty ( Not literally ), but example if I have a client who does not have a lot of gym muscle I will let him do Squads, Bench, military press do "build" more muscle.
See those exercises as your clay you pack on more clay. Now, now we want to shape this clay into a sculpture.
So you need a chisel to carve the clay, and to me dumbels are your chisel it's your tool to transform your very anatomy of the muscle focusing on fibers otherwise overlooked.
NOTE:: The above explanation does not apply to experience gym people. Both dumbels and Bench has it's own unique synergy between them need to be utilized.
However, for someone who just started Squad, bench, military press = MASS, Dumbells = Athlete body
Do not read to much onto that. disclaimers for days.