Can masterbating alot be the reason my left arm looks slightly bigger/more muscular than my right? it looks like more muscle tension

Possible: Any repeat action may cause a muscle or muscle group to enlarge. Plus most of us have a bit of muscle asymmetry because of having a dominant hand. You can also injure yourself with too much or too vigorous activity from penile abrasions to tendinitis in your hand or wrist, so be careful.

Unlikely: Most of us will have a subtle difference in arms/forearms with the dominant arm achieving more muscularity. When it is used more to reach, select, open doors/steady balance it will be slightly larger. Rings on that hand may also size a bit larger. It is the mechanics of all you do in life, not a single function.