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Is your D7100 a Nikon?
If so, it, not the lens (unless it is a Zoom lens) is where the magnification (otherwise known as 'digital zoom') would take place.
Try using the Quality/Zoom In button on the D7100 (if it is the Nikon brand of D7100.) That may help... though whether that enlarges the resultant picture depends possibly on the lens assembly attached to the Nikon D7100 camera.
The lens you name, the 55-300mm, if it is the Nikkor VR model, is a Zoom lens, BTW. So using Zoom controls on your D7100 should actually zoom subjects e.g. birds.
Does that work? let me know, please. And I will advise you further, as needed.