easy fix, leave the cap off for a week or so, it will shrink back down. after you get it where it works good again lay the saw with cap up in the future. or buy another cap and rotate them. there cheap.( i type to slow)
Buy another cap and swap out as needed, stay away from etho gas seems to help also. Replaced both caps on one I own two years ago and haven't had a problem but I run etho free gas also.
It's the alcohol in the gas that causes the caps (and other things) to swell. Most premium gas up here in Ontario is alcohol free but that may not be true where you live. As mentioned above, leaving the cap off, preferably out in the sun for a few days will restore it to size.