Oddly, i'm not using any barbells at all, all i'm using are dumbbbells. I'm guessing that the muscle on the right is bigger because when i do arm curls with left hand, i'm sitted and bend forward with right hand putting on thighs supporting the elbow of the left hand that i'm using to do arm curls(hope you get what i mean). It might be the case that when i'm running out of strength, i tend to use back muscle to force myself to lift it up, hence the trapezius muscle on the right is bigger than the left as left hand is weaker than right, so i used more force from the back to force myself to lift it up during the last few lifts.
Moreover, although its the trapezius muscle on the right that is bigger, but it seems like its not really the trapezius muscle that links to shoulder, instead its the BACK TRAPEZIUS(i don't know if theres such a thing). Meaning when seen from the side view,the BACK trapezius muscles tends out "pop" out more than the left.
@Boss man: I had a similar problem in the past with calfs' muscles, again, the right calf was larger than the left calf, so i started training ONLY the left calf. One day i found out, although i'm only training left calf, i was training the wrong muscles as i put too much pressure onto the left leg and it sort of went... idk..In the end, from the front view, left calf muscles seem to be WIDER(in terms of the width) while right calf seem to be wider when SEEN from the SIDE. Hence i don't know what if i should just train solely on left trapezius as i'm afraid the same thing might happen.
I hope its understandable as i'm really lost about what i should be doing now.
PS: Sorry for the wall of text!