i have symmetry issues too.
left shoulder, pec, and tri are bigger
right bi and quad are bigger
it is most noticeable in my delts though. the problem started in high school - threw shotput a year (lefthanded) and that same year hurt my right shoulder. haven't been able to correct it since.
i tried to do extra reps for my right delt, or focus real hard on form with that side, or tried to conciously carry plates with my right arm. in truth i think i did more harm that good. i was constantly reaggrivatin the injury (even without the extra reps at times), which probably kept that side from growing. i think my best bet may have been to lift each side the same, focusing on isolation movements. what makes things hardest is that my bigger muscle isn't necessarily stronger. my smaller right shoulder is stronger, and my smaller left bi is stronger. i guess it's compensation - neural recruitment for strenght to make up for the difference in muscle size. either way it just complicates things.
unfortunately i don't have any advice for you. instead i have a similar problem and need the same sort of advice you do.