I have the same problem. My right is larger than the left, i always try to hide it, my friend pointed it out the other day and i felt like **** the whole day. but it will even out itself so keep lifting.
I actually have the same problem myself...
My right arm measures 18.5 inch and left arm 19 inch.
It's weird because when I lift weights it feels that my left arm lifts the weight in a completely different fashion(perhaps due to my brain?)... I can 1 rep a heavier weight on my left arm but I'm able to do more repetitions on my right arm.
Sometimes people over-train one arm so the other arm may be bigger... Another theory is that if you're using your right arm very often, maybe that could be hampering your recovery and thus resulting in the left arm(which is able to recover fully) to be larger.
I'm 5'9" and 273lb, endomorph.
Another thing for me is that as an endomorph, I don't require as much sleep as people of other body types. My wrists measure 8~ inches I believe so I guess I am rather big boned but I've only been training since I was 13 and I'm currently 17. Ever since I first started lifting weights I had this awkward feeling when I'm lifting weights on my left arm...