Width of text in mail

Hi,
I ask this here, but I know it’s not specific to Kmail or even Linux.
If you type something in Kmail, the text-block is much smaller then the space you have. Apparantly there’s a fixed width. You also notice that when you see a reply, that’s a lot smaller than the actual space you have.
In a lot of mailprograms, you can type in wider space then that reply (in Mail on Mac of in Betterbird, for instance).
What’s the protocol around this?