I just thought the button had grown. I didn't recall it not having a border. But it feels like not only is the button "bigger", but the font looks bigger, too. (I'm guessing that's a perceptual illusion, though. I expect that the Search font was just about the size of the text box to the left, so when the border appeared, it all looked bigger.) I suspect that had the button size been about the same height as the text entry box, I might not've noticed the change. It all looks good, though, no complaint--just curiosity.
Just for reference, I'm using the 'Blue' theme, I'm on a Linux box (Ubuntu) running Firefox. (Probably irrelevant since there's nothing wrong.)
When your only tool is a hammer, all problems look like your thumb.