There's a couple of ways a crankset can be stiffer, given the same quantity of the same materials, and the simplest is to have tubular (hollow) arms and bottom bracket. If you take the same cross section area of that old XT crank arm, and make it a tube, it's going to be stiffer. If you take the same BB spindle and make it a tube, it's going to be stiffer. Even a cheap crank now has a C-shaped section instead of a straight one like the old XT to add some torsional stiffness without doing any really clever manufacturing.

With better quality materials you can make the tubes ever-fatter with ever-thinner walls, getting the same performance with less and less mass. The chain rings that look like disks are hollow too.

I was lucky NAHBS was in my town this year, and Shimano (a sponsor) had a nice display of an entire alloy crankset cut in half to show the hollow arms and spindle. I can't find a photo of it, but it was neato. It wasn't obvious how they'd made it.

This article has a nice photo of a modern crank arm cut in half to show its features
http://www.bikerumor.com/2016/03/24/sram-eagle-12-speed-mountain-bike-groups-let-you-soar-far-and-wide-complete-tech-overview/


Here's a Hollowglide chain ring cut away
http://www.spoke.ie/2009/04/new-shim...egra-groupset/