What version of Premiere Elements are you using and on what computer operating system is it running?
What are the properties of the souce media on the project's Timeline and what did you or the program set as the Premiere Elements project preset?
When you got to the upload to YouTube within your version of Premiere Elements what did you select as the preset there?
What screen is being only 1/2 filled, your computer screen when you press the Full Screen button at the YouTube playback site or the non Full Screen view that at the YouTube site?
Let us explore the project and upload settings.
We will be watching for your follow up.