Uh, if the bezels were smaller then you would be able to reach more screen real estate. Since the entire phone would essentially be smaller.
If I adjust the gripe my thumb touches all parts of the screen perfectly. The same grip on a 4.7" phone and I cannot do it. It has everything to do with the screen size and nothing to do with the bezels. This sums it up perfectly. Notice the bezels are irrelevant.