When looking at the evolution of the straight razor as far as we can go with correct provenance of dating a razor with a maker, you find that razor started in the 5/8 region, and wedge grind. With the advent of hollowing it seems like the razor are available in bigger sizes.

Like Edgewitt mentioned a Barber honing a razor frequently would want a larger size razor that will last longer, sounds like a plausible theory to me anyways. Going further with the more precise hollowing, to match the heft you need more length.
That said i dont have a preference in size or grind, its the beauty of the hobby to try different razors.