the whole point of synthetic and wool clothing, is that it wicks away moisture, and drys fast. If you wear cotton over your synthetic, the cotton will still stay soaking wet once it gets wet. If you aren't gonna get wet or sweat to much, it's not really a big deal, but if your shirt is gonna get wet for any reason, cotton is gonna suck, and could be dangerous.
P.S. you can find looser fitting synthetic shirts. I routinely were a short sleeve synthetic over my long sleeve when it's cold while backpacking or mtbing. And I wouldn't worry too much how you look, it's better to be scrawny in a compression shirt than a big fatty stuffed in a compression shirt.
P.S. you can find looser fitting synthetic shirts. I routinely were a short sleeve synthetic over my long sleeve when it's cold while backpacking or mtbing. And I wouldn't worry too much how you look, it's better to be scrawny in a compression shirt than a big fatty stuffed in a compression shirt.