Why is it believed; in a universe 10^23 times greater than the one observed, it remains geneous throughout?

The universe appears to be geneous only at huge distances, on the order of mega parsecs. I've heard even larger estimates of the size of the universe, something on the order of 10^300. At that point units of measurements begin not to matter, so that the difference between micro millimeters and mega parsecs are minuscule by comparison.