Also, could someone explain to me why the 30mm (bigger) are lighter? https://www.sram.com/sram/road/products/sram-red-crankset-0 (similar with rotor)
30mm lighter because it can be made from aluminium. 24mm needs to be steel.
The new Rotor Aldhu is a mix-and-match system where you select different pieces to assemble the crankset you want; arms, axle, and chainwheel set. Since it comes in both 30mm (7075 aluminum) and 24 mm (niquel coated steel) axle versions, it's possible to isolate the weight difference of the just axle. The single available 24mm axle is 145g; the same width 30mm axle is 96g. The drive side of the two systems are identical, so this is as close to an apples-to-apples comparison as you can get. FWIW, the crank arms for the 30mm axle are 8g less than the same length arm for the 24mm axle, presumably because more material has been removed to make the larger "hole" for the larger axle on the non-drive side arm.
"They're made of latex, not nitroglycerin"