Its not less work really... its less work compared to the video card.
So on a scale of 1 to 10, right now, your cpu is running at say 6/10 and the gpu is running 4/10
Getting a bigger monitor, higher res, the gpu will have to run harder so it'll bump up to 8/10 with the cpu still at 6/10.
New cpu will get the info to the gpu faster, so it'll run at say 4/10 with the gpu upping cause the game runs nicer to 8/10 again
New gpu will allow the data to get to your screen faster and better, so say 4/10, but the cpu is gonna struggle to spit the info to your gpu so it'll up to 8/10
As you see, you open a pipe that one of the old peices struggles to use..
Its abit confusing, but hopedfully i helped abit.