I've been editing my I3 footage on final cut pro X using the standard color space of rec 709; however, this automatically makes your prores raw footage dim lacking contrast and limits the amount of colors that you can use for color grading.

In order to preserve your raw files and keep the highest possible resoultion and contrast and you must modify your color processing settings to Wide Gamut HDR before beginning your project.  The footage looks amazing but the problem is when you try to render it in 8k a few seconds clip turns out to be 60 gigs! and if you try to render that project in 4k then the footage appears dim lacking contrast again and you might as well edit the project under the standard rec 709 color space if you do that.

My question to you guys is how are you able to get the highest possible 8k 60 fps image with the wide gamut HDR color space without having a file size that ends up being over 500 gigs!

export it with h265 codec. It supports 8k, 10bit, 60fps and hdr at smaller sizes.