All I can figure is my nice big thumbnails must have been a carryover from an older version that had that ability.
Always before, instead of three smallish thumbnails per row, I had two sizable ones and could quickly switch that to a single large one, much like the Preview, except that the image information displayed and there was no strip of thumbnails across the top.
I have investigated every option in every menu, right-clicked, control-scrolled, and just about everything else I can think of to do and I cannot manage to get my two per row nice easy to see thumbnails.
Sliding both sidebars to their narrowest still only allows three per row; and, once those three reach a certain size, it only widens the empty space.
As best I remember, when I right-clicked Thumbnails, there was an option to set and choose thumbnail size; I don’t have that option anymore.
I never before remember even messing with that slider in the bottom-right and did not know what it did until I went on this quest to get my big thumbnails back.
I am fairly certain that I have screen-shots showing the two big thumbnails, but I can’t figure out how to post them here or even if that is possible.