This is running "zoom". When I try it in Finder, it doesn't make the window full screen, it actually made it smaller.

When I use the Window menu, Zoom replicates what double-clicking the top title bar does, while Fill maximizes the window. This holds true with the behavior you describe in Safari as well.

It just seems like a lot of apps treat Zoom and Fill the same now (I tried Calendar, Notes, TextEdit, and NetNewsWire), which adds to the confusion.