File size of captures has zero to do with the CPU. The CPU is more about speed of compression than anything. The file size of the capture is all about the codec you use, and the settings you use in DxTory.
Outputting the file to 30 FPS, especially at 720p, will definitely reduce file size. If you only want to play at 1080p60, and output to 720p30, that is a smart way to go for that level of GPU.
It should also help keep your PC from lagging during recording. That said, if it DOES lag more than you expect in some places, you can use the DxTory codec and output to RawCap vs AVI, and use more than one HDD to write the RawCap files, which you convert into an AVI file via the RawCap Convert tool.
The only caveat of this is the DxTory codec in uncompressed mode writes HUGE file sizes, but you can use Med or Low compression mode.
That said, with your rig I would just use ShadowPlay. It's convenience is unparalleled, the capture quality is decent, and the hardware capture is so fast and optimized you can literally output at 1080p60, and still get pretty small file sizes.
ShadowPlay is ideal for a game like GTA V, because you never know when you're going to have a moment worth capturing. You can just hit shadow record after any 20 min segment you decide you want to keep, and record fast and lag free.
Same spec as yours @ 1080p60 output file via ShadowPlay
[video="https://www.youtube.com/watch?v=3OGnYbjG96I"][/video]