I'm confused by your pec being smaller but the Triceps of the same side is bigger. I'd have to see an actual bench press on that bench.
For what it's worth my right pec looks different than my left. When I flex it has a different shape, possibly smaller. My right arm is bigger. I'm right handed. I just chalk it up to having a dominant side and having no control over the fiber alignment and origin/insertion of my muscles.
It's kind of like abs--they're not always perfectly symmetrical and no training will change that.
Perhaps 6 months of dumbbell bench will tell you if it's just your body or if you're actually out of balance.