kirnale

so, it could be from the fact that the GPU is doing a lot more than it used to, allowing for some work to be taken off of the CPU. Also, when you crank up graphics, it takes work off of the CPU and dedicates that work to the GPU. 
 
It works like the same way for me, where I use 100% of my CPU when playing overwatch at 100% render. when i take it to 200% render, cpu drops to 60%. Gpu, well... the 2080ti speaks for itself at 1080p LOL it doesnt go above like 15%. 

My GTX 780 runs Dx11 without any problems, I'd assume the problem with the GTX 770 was that it was a somewhat weak card to begin with, but 2GB of VRAM really isn't that much.

That's strange indeed. When i changed my gpu, my cpu usage went from 60-70 to 100% while new gpu's usage won't surpass 30%. But you had 100% cpu usage with the old gpu whereas i didn't in my case, so there might be a chance that your new cpu is relieving some stress off from your cpu.

600/700 series are full DX11 cards.
 
Its more likely that CPU usage was due to texture swapping like your first guess.  It could also be due to the Kepler shader compiler/command dispatcher in the driver being more CPU intensive.

It can be various reasons.
 
The video game may have used more than 2 GB of video memory, so the game engine was busy more often swapping data between video card memory and ram.
 
Also, the GTX 770 is an older card and it could be the game defaulted on a DirectX9 or 10 renderer instead of DirectX 11, because some DirectX 11 features could have been too gpu intensive on that older card. Maybe the game engine was also busy taking higher quality textures and always resizing them to lower sizes to fit into lower quality presets, to squeeze more stuff in 2 GB of video card memory.
With the newer card, the game may run with different rendering paths, maybe different texture sizes, maybe the driver for the GTX 1xxx series has more multithreaded stuff, maybe the game can use some DirectX 11 features for more parallel processing...
 

task manager. 

Not having to go and transfer information as often does help reduce the load on the CPU but it shouldn't have a massive effect.