You can double click on any part of the top title bar (that doesn't have buttons in it) for example in Calendar you can double click beside the magnifying glass in the top right and it will maximize the window.

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.