The difference in weight seems trivial to me. Looking at XT centerlocks:
140mm= 88g
160mm=108g
180mm=132g
It's also worth noting that the Shimano road rotors are actually a bit heavier. I'm guessing because they have larger cooling fins? The Ultegra centerlock rotors are listed as:
140mm= 108g
160mm= 128g
I've never understood why anyone runs 140mm rotors. On the flip side, I'm north of 75kg and have never felt like 160mm rotors were problematic, but if I were barreling down mountain descents I could see wanting to try 180mm.