I grow cherry size tomatoes on my deck in a pot. Why are the skins har

?? what about the skins? are they splitting or are they tough?

Some cherry size have thicker skins, my grape tomatoes have a thicker skin than my sweet 100's or yellow pear. The thinner skinned ones have more water in them and will split easier with a large influx of moisture, whether it is rain or watering after is a little dry. If this doesn't answer your question, please reply with what the question about your tomatoes are doing since your question got cut off.

Usually to much hot sun

According to gardeningknowhow.com, It could be from under watering or from high heat. The high heat causes the skin to thicken to prevent scalding of the tomato.