MPEG4 is generally a highly compressed video format designed to be viewed, not edited. Premiere Pro, therefore, does not support editing MPEG4 files.
The reason the file was so small is that it is probably spatially as well as temporally compressed. Just for reference, a MiniDV camera uses approximately 12GB per hour of video.
I understand what you want to do, but using the Internet for these files is causing you pain you would not have if you had the original material instead of highly processed clips.
Sorry.
Steven
https://www.stevengotz.com