At least for the "big boy" lenses, back in the 70s Nikon and Canon started making a serious effort to fix chromatic aberration in long, fast lenses. Nikon started using "Extra-Low Dispersion" glass to fix this. It worked great and was really durable, but heavy. They were proud enough of it to start putting gold rings on the barrels of lenses using it.
Meanwhile, Canon figured out how to grow and polish huge Calcium Fluorite crystals as lens elements. The results were superb and the fluorite element is very lightweight, but could also be delicate and, probably most significantly, had a lot of focus drift as temperatures changed(more so than glass). They were proud enough of it to paint a green ring around the barrel to show it off. A few years later, along with a "refresh" of the lens mount design, they started painting Fluorite lenses white supposedly to make them less resitant to heating up in the sun(and also make them instantly recognizable), and then adding a ring of red paint and the L designation on any lens with either an aspherical or fluorite element.
For comparison on these early lenses, the Auto-NIKKOR 300mm 1:2.8 ED was 2.6kg(5.72lbs) while the Canon FD 300mm 1:2.8 S.S.C. Fluorite was 1.9kg(4.18lbs).
Incidentally, the gold rings stuck around to show ED lenses at Nikon, and then later when even $100 kit lenses started using aspherical elements and ED glass to designate high end lenses. Canon started using the red ring to mark a high-end lens even though a lot of lenses at all price points now use aspherical elements.
Nikon finally caved in and started using fluorite a few years ago. The one that comes to mind for me is the 70-200mm f/2.8E FL, although I think one of the exotic super-teles used it first. If you compare the 70-200mm f/2.8E FL to the 70-200mm f/2.8G VRII, the E FL version is about 100g less(1425g vs. 1532g) despite being a 22 element lens and the older lens 21 elements. Nikon never made an F mount 300mm f/2.8 FL, so it's hard to say if it would have been lighter. If you go out to the 400mm f/2.8, the newest non-FL is 4.6kg and the FL is 3.8kg.
BTW, to go back to the 70-200 zoom, Canon's newest/most current EF mount version is 1.6kg with the tripod collar(the collar isn't readily removable on the Nikon, but it loses another 70g if you take the foot off).
With that said, none of this plays into a 10 year old thread on the D5100 and 600D. They're both great-buy the one you like better. That's still true, especially at the bottom end of the market. Actually a lot of it's still true at the top end of the market too unless you get into a fringe case where one brand offers a certain lens that the other doesn't, You can split hairs about how one flagship body might be better than another for AF speed, noise, frame rate, or whatever but it's pretty rare that one brand has a make-or-break feature that the other brand doesn't have.
. And obviously, that's the sort of question you could ask the tool.