This (space efficiency, not necessarily the top menu bar) is the main thing that's keeping me away from Linux at this time. "Liberating" a Macbook Air feels like a great idea right until you have to do actual work on it, at which point, unless you're using the terminal, the screen will be largely empty space and huge widgets, with whatever text or image you're working on cramped between all those beautifully hand-crafted organic breathable widgets.
Edit: I'm not talking about window decorations being large, I'm talking about application widgets being large. You can fit like three macOS buttons in the space of a single Adwaita- or Breeze-themed button. See e.g. this SE thread for a comparison -- not with macOS, but basically the same problem: https://unix.stackexchange.com/questions/489533/how-to-get-a...