Re: 610 vs 7100
Because the DX sensor is smaller. The image projected from the lens is smaller on that DX sensor. (if the same lens, the image is the same of course, but that cropped view is not. I'm instead speaking of the same the field of view, made to be the same image view.)
The number of pixels could be the same number (like 24 megapixels), and if thought of that way, then printed at X dpi would seem the same thing. But that is only about pixels, and NOT about the image they represent.
But the printed image is in fact a reproduction of the original lens image, and DX is simply smaller. If printed same size, the DX is enlarged more (FX could be allowed to be more blurred, so to speak - if not enlarged as much). This is why CoC used in DOF formulas is 1.5x larger for FX than DX. The larger FX is not enlarged as much. And it is why DX is better than compact cameras.
It's the same reason large film was used, like medium film for weddings, or sheet film for commercial work. It does not have to be enlarged as much. Analog, but considered a strong and obvious plus.