Yeah, but what does it really mean to "fit the content", if it doesn't fit the window at any size, but can reflow horizontally - as in e.g. most webpages? Mac apps seem to prefer increasing the vertical size until the window is as tall as it can be, but for horizontal, they usually stop before they reach full screen width - even if this would fit more content on the screen.

It's supposed to be "smart", in a sense that it gives you the largest window size that it considers usable. But that's too subjective an assessment to enable it by default, and even more so to not make it configurable at all.