Quality after exporting is worse and file size is much smaller than estimate

Hello, my preview video for my project looks good but after exporting the quality becomes awful. I select my settings to export to 4K with 29.97 FPS but the final size looks very pixelated and the file size is around 300 MBs, which is way to small for a 3.5 minute look 4K video. This is very weird and if I can't find a solution I will obviously have to find a different program.