PD always fills the frame on two sides and will leave black bars along the other two edges if the media isn't exactly the same aspect ratio. Typically you'd use the PiP Designer to zoom in to fill the frame, or maybe levae the clip as-is and add a blurred backgound image or video to fill in the blank spaces.
Depending on how close the clip is to fitting, you can also try right-clicking on the clip and then try
Set Clip Attributes >
Set Aspect Ratio Settings and see if PD can automatically stretch it without messing up how it looks too badly.
You could also take a screenshot of the whole timeline + preview screeen so we could see your exact situation. Use
Alt Print Screen or the Windows snipping tool (
Win+Shift+S) then save the image as PNG or JPG. Click on the
Attachments button below the forum's text box to upload and share it here.