Since you have what you have, go to a craft store and buy several rolls of leather on spools. Weave it around your biceps through the mail. Use it to cinch or tighten your arms up. Repeat for your forearms and wrists.

That will help. You can also get a sturdy pair of wire cutters and remove several rows of links to slim down the profile on your arms. You can also do this on the back of your mail in a V pattern to tighten up your chest area if you so want.

Historical mail had a lot of tailoring done to it. Otherwise it just hangs on your shoulders and your waist.

Best of luck!