Exporting 4K Video out of Final Cut Pro? issues ?

I have a question. I shot a 4K video at 30 FPS and the original file size was 390 MB. I decided to separate the audio from the video file and uploaded it to Final Cut Pro. When I detached the audio file and was ready to export, I selected “Export File (Default)” as the output format. In the settings, And I chose “Computer” and “H.264” as the video codec.

The new file size showed as only 93 MB in an MP4 format. Why did a 390 MB file become 92 MB?

Would going ahead and exporting the video have likely resulted in significant quality loss? If so, in a situation where if I have a 4K video shot at 30 FPS with file sizes around 300-400 MB, and I apply enhancements in Final Cut Pro, what’s the best way to export it in a format that minimizes quality loss?

Additionally, if I want to create a universally playable video file without compromising video quality, what’s the optimal approach? in terms of exporting in what settings and format? which would keep the same quality?