I bought a ThinkCentre tiny, which is a desktop about 7"x7" size, smaller but thicker than a laptop. The whole machine gets really hot, especially the bottom, much hotter than my Thinkpad.
I thought even though it's small it's still a desktop, so should be cooler than a small laptop?
Uses more power. Possibly the fan is full of dust bunnies?
Considering it runs one fan and dumps a ton of heat out, it'd naturally run hotter. I wouldn't worry about it unless its starts throttling down which usually means its getting hot.
It's a brand new PC, no dust, not surrounded by books.