after exporting, video quality is poor unless i zoom in

i realise my question might be a bit awkward, but i'm a total noob at this so any help is appreciated!

im making a video for my friend and have a bunch of his ingame replays; all mp4 files, all 1920x1080.

but when i export it, even at 1080p highest quality, the picture is consistently pixelated and jagged, unless the video hits a section where i've zoomed in.

at that point, the quality improves dramatically and the pixelation is completely gone 90% of the time.

how do i fix this? are my computer's specifications a factor?

thanks!