Can muscle shrink by injecting it with some kind of substance like botox?

No.

nope..dont think so

You can’t inject it, but if you put it in a sling, the muscles will atrophy over time and your arm will shrink.

I’d just switch to using the other arm if you know what I mean.

I’m with @Winters. Why shrink the stronger one? Work the other one more so its muscles build up to match.

Besides, Botox works by paralyzing muscles. Fine for your forehead, because it doesn’t have to grab, pick up, or hold onto anything. Your arm? Probably not…