It simply means that the dimension of your source material, your sequence settings and export settings do not match.
If you put a pea on a tiny saucer, it is easy to find, if you the same pea in an Olympic swimming pool, you will be hard pressed to find it, because of different dimensions. Same with your workflow.
Welcome to the forum.
As Harm states, this is due to a mis-match between your source footage, and your Project/Sequence Preset.
What are the full specs. of your source footage? Your Project Preset MUST match that.. If there is not a Preset that does match, you can choose Desktop and then adjust the parameters of it to fit your footage.
When the above is done, you can Export to all sorts of Frame Sizes, and things will be fine, but you need to match that Project to the footage.
That will handle it.
I am having this same issue but all my settings are in line as far as I can tell.
Source material is 640x480. Project setting is the same, and I'm exporting to the same resolution. Then final output has the video as a small portion of the frame. If I just go and view the original source video this is not the case.
Post screenshot of export settings with the left tab to output with a visible image.
On what device or with which viewer are you watching the end file.