Rashguards are supposed to fit like second skin.
When wet the fabric will extend even more and the fit will loosen up.
They garment should fit tight so that it helps you to move more ergonomically in the water and won’t be in your way.
Rashguards are worn under wetsuits, in this case you also need a tight fitting garment.
If you don’t like and don’t need tight fit please feel free to order a size up.
Please remember that there is a silicon line on the bottom of the rashguard that might feel uncomfortable if you wear different sizes for the bottoms and tops. For example if you wear Top size S and have wider hips so you usually buy M size pants, then the silicon line might restrict your movement and I would suggest you to buy M size of the top as well. Unless you need your rashguard not to move and to stay in place . rashguards fit very similar to wetsuits. If you wear an S size in rashguards then you should go with the same for wetsuits and leggings.