macOS handles the game window size based on the desktop scaling. This means that the client resolution can be rendered the ‘same’ by the client, but be different sizes depending on this setting.
Eg, in these screenshots the client resolution is the same but the desktop scaling is changed:
One is taking up a little over 1/4 of the screen space, while the other is effectively a full-screen windowed mode.
There are really good reasons macOS does this, but it can be unusual for things like games. We’ve been discussing this internally and with Apple, but there isn’t a perfect solution right now.
Edit - there are tools that may help with this that some people found useful in the playtests: Native Mac Client Playtest - General Feedback - #118 by Ender_Tunin
We have to be careful with default behaviour, so we’re letting the OS handle it currently.