Video playback is handled by a small hardware block running at 250MHz.
Kodi’s UI is handled by 4 large arm cores running at 1200MHz.
One uses a lot more energy than the other.
Unfortunately Kodi’s UI is not very efficiently written. The whole skinnable approach, whilst being very flexible makes it quite heavy to implement.
But really you just shouldn’t worry. The electricity consumed by a Pi3 is negligible. Running a Pi3 at close to 80’C is perfectly normal and won’t cause any issues.