Depending on your rendering method bit rate/ codec/re-scaling/etc... all those influence hard disk space
The higher bit rate of video the better quality on youtube.
This reminds me 4k gives you better bit rate on yt.
Here some recommendations for yt. https://support.google.com/youtube/answer/1722171?hl=en
Did you add a lot of things? If you add overlays, text and other effects, the file becomes bigger. The program can also render the video in a higher quality then the original file, upping the file size. As Laitz said, you will have to check what size and file type you are rendering on.