Zoomed medium UTH too big for 20 gallon?

I accidentally ordered the medium which is for 30-40 gallon aquariums. If used it would be nearly half the surface area if the bottom. I am using a 100!w Che and a small UTH both on thermostats. Is this too big?

with a good enough thermostat it should be no issue. maybe the surface area is a bit large, but when the thermostat does a good job i do not see a problem in getting the temperatures right.

I'm afraid it will make too large of a basking area.