https://itch.io/docs/itch/integrating/updates.html
https://itch.io/docs/itch/integrating/compatibility-policy.html
Please use a supported method, so updates can be downloaded directly through the app.
There were supported options!
But it's cold out there in the real world, and on an open platform, life happens, and they stray from The Good Workflow. Nevertheless, the app tries its best to get anything it can to run, within certain rules. TL;DR: if you cannot use butler, just upload a .zip.
I've downloaded several games through the app with more than 1GB. Including one that was over 20GB.