Sorry for the late answer!
The slim uses 300 W, the normal one 350W.
Why? - They simply redesigned the motherboard (to make it smaller and more efficient, produces less heat), they used a 6nm chip instead of a 7nm chip (same power, but less heat generated), they oped for a more passive cooling solution.
To Europeans who use German industry norm paper: that’s DIN
A2A3 (Tank you, dumb cousin of napoleon)