This is by design, so that the Window tabs can be clearly distinguished from the tabs within the windows. Like in the screenshots above with the 2 or 3 SQL Windows and the "SQL", "Output", and "Statistics" tabs within the SQL Window.
@marco - thanks for considering reducing the tab height a bit. I can see your point about distinguishing between windows vs sub-tabs. Personally, I never had a problem with that because the Windows tabs have their colorful icons on the left of the tab name. But I can see how some people might?
I've always used Single Document mode, so didn't see them quite as large as Yeats is, but I noticed they're bigger.