4 possible causes, the first of which is the variety of tomato, some just have thicker skins. That isn't the case with cherry tomatoes, so the next 3 things are these - too much high potassium feed, so if you've not fed at the recommended rate, that might cause it. Otherwise, not enough water or not often enough - the skins thicken to conserve moisture, or too much very hot sun - skins thicken to shield themselves better.