Yep. Upgraded July 29th, noticed the increase SSD space after August 29th.I'm assuming you upgraded to W10 more than 30 days ago. The rollback to 8.1 is supposed to be available for 30 days and whacking that would seem the logical answer to your observation.
But back to the thread's topic, getting hot while supposed to be off, or in standby or even hibernating, turning on while off, etc. seems to be an issue with Surface, not just Win10. There are lots of threads here about this sort of misbehavior. Seems to be a Surface+Windows standby (or connected standby) and I hope Threshold 2 fixes it.