Sorry, I know I'm being pedantic, but you are using dpi wrong. I think you mean 720p and 1080p. Those are descriptors of the number of lines of pixels on a progressive scan display. Dpi is dots per inch (interchangeable with ppi, or pixels per inch, on a digital display) and is arch lower figure on TVS (because you are sitting for back, so you don't need it to be high.)
A 37" TV (roughly the average TV size) has a ppi of 40 at 720p or 60 at 1080p. On a 4K screen the ppi only goes up to 125. The original iPhone had a ppi of 163, for comparison (back in the pre-retina days)