@Biggles so the mastodon default is 500. QOTO runs a modified version of the software with a limit of 65k. However, the app I think you're referring to is published by the official Mastodon team, which takes the position that Mastodon is for *micro*blogging and if you want to write longer posts you should not use Mastodon. They have at other times resisted making changes to the interface to accommodate longer posts, so it wouldn't surprise me if this were a deliberate ideological choice to maintain Mastodon's "purity" as a microblogging platform.
Most interfaces hide everything past the first 500 chars behind a "show more" link or button, as everyone's kind of come around to the idea that longform instances exist and you need to accommodate their toots in a way that doesn't break the interface, even if you don't permit them yourself.