Here’s a weird one - my friend Scott and I both have MacBooks, bought a couple of months apart. On his laptop, the Apple logo at the left-hand end of the menu bar and the Spotlight icon at the right-hand end are slightly, but noticeably, smaller than on mine.
I’m sure there’s a simple explanation for this, but I have no idea what it is!
UPDATE: Thanks to Flickr user MacGeek Pro, we have got to the bottom of this. It turns out that the small menu bar items were a feature of some versions of UNO, a sunken unified OS X theme. I had entirely forgotten that my friend had this installed, but even so it would never have occurred to either of us that an app designed to create a consistent look for OS X windows would have any effect on menu bar icons. You learn something new every day!
I have uno installed and my icons are like yours… how odd.
Apparently it was only certain versions of Uno that changed the icons. Odd indeed.