Why are my youtube videos not fullscreen when i use my nikon d3100?

Whenever i shoot a video with my Nikon d3100 and upload it to YouTube, it doesn't appear full screen. Is it something i can change in the video settings or can i change it with something else?

Phone is too small.

Use a movie editing software such as windows movie maker to crop it to the correct youtube dimensions

What happens between shooting video and uploading to YouTube? If you don't know then the answer is there. You must control the conversion yourself. YouTube will always make it smaller to run faster.