Uploaded gmod videos have bad quality

I’ve recently started making stop-motion videos using gmod. I bring the screenshots into Windows Movie Maker and make a sound file in Audacity, syncing it as I go. I upload the final product to YouTube and the default quality looks horrible (even though it looked fine when viewing it in Windows Movie Maker) and it must be set to HD in order for it to look decent.

Additionally, when I sent the file to somebody for a gmod collab, the received file was in low quality, while the parts submitted by other people were fine.

I’ve tried changing the file type before uploading it, and I’ve tried multiple uploading methods. Nothing seems to work. I’m sure that the video must have the POSSIBILITY to be uploaded in good quality since it can be viewed in high quality by changing it to HD, but I’d prefer for the default quality to be good.

Is this a problem with Windows Movie Maker, or am I doing something wrong?