I suspect the size hung around for a while for a couple of reasons.

The physical size (on screen) didn't vary by much. If I recall correctly, 72 to 120 dpi dominated until the introduction of Retina/HiDPI displays, and 120 dpi was pushing the limit since scaling wasn't really a thing. (When it was a thing, it tended to be handled by applications where it was important, such as desktop publishing/graphics design applications, and it only extended to the content area.)

Add that to the purpose of these buttons: they were intended to be unobtrusive messages about which browsers were supported, or which browser the site owner prefered. Going bigger would not have much of a point. (That said, they were as obtrusive as heck to those of us using unsupported browsers!)