Context : On most of my documents, with few but very distinct colors, I'll often switch to "big thumbnails". It's easy to distinguish a color from a lighter or darker version.

But then, when the panel could displays two lines of swatches, I would only need one (see linked file), freeing a lot of screen space.

Except the minimum size of that panel doesn't go under 2 lines, when 1 would be enough.