Motor Heat 1515 vs 1512

I run both a 1512 3d and a 1515 2.5d on 5s in my RC8. I pretty much just race the car. I have noticed that the 1515 runs a little hotter in my buggy than the 1512. I chose a 1515 because I thought it might run very cool because it won't have to work as hard. People use 1515 in truggies and they seem to run pretty cool, so I figured it would be even cooler in a buggy.

After a 15 minute main, the temps on my 1512 are usually around 150-60 degrees. The 1515 seems to run around 170ish. The temps don't really bother me, but really just curious why the larger motor would run hotter. What gives?

I always gear my cars to usually run about 40 or so mph.

The only thing I can think of is that the 1515 doesn't have enough load on it. The MMM ESC is usually around 140 degrees after a hard race. I guess I don't understand why these motors run hot if it isn't working hard enough. It would seem heat would be an issue if the motor is working too hard, not the other way around. What am I missing here?

I can't really gear the motor up because then the car would have too much top end, although I guess I could turn the EPA down a tad.

Any thoughts?