Here are screenshots for all five text sizes (I put the text size in the filenames). As you cycle through them you can see that in fact all three buttons are not right. They're all oblong at small text sizes and square at L. The middle one is the odd one out at XL. This is my 12" Macbook. I'm using an uncommon resolution of 2048x1280 courtesy of a third party app. I checked it at the default resolution of 1440x900 though, and the buttons behave about the same.
