40 times the original filesize?

hi, noob here.
im just testing the free version atm, but my first few renders are horrible, not in picture quality mind you, that is quite ok.
but i cant get it to use my GPU (3080) and the mp4 files i rendered came out with about 40 times the original filesize? what the hell am i doing wrong? i have 5 TB of files to render and sure as hell dont have 200TB of spare hdd-space, lol.
source is around 2500x2000 mp4.h264 upscaled 200% with artemis low quality.

should i use another output like h265? and how do i get it to use my GPU? as far as i can tell it only uses the VRAM of my gpu, letting it idle at 0-1% and uses about 50% of my 5800x CPU.
edit: just in case, yes, the gpu is set as renderer in the settings.

If you are exporting to MP4 H264, what number did you set in the Constant Rate Factor ?

had to check, that is set to 20. no idea what it does.
edit: also my 20hour render(settings in the OP) just completed, it bloated my 1,1GB source up to 48GB… that is just nuts.
realy hope someone can tell me what im doing wrong.