I believe the Signal desktop app is an electron app. That’s probably why the updates are so big, has to update the bundled browser.
Because, instead of "trusting" the update (or rather codebase) of a messenger, we now have to trust the complete browser bundle.
You may additionally ask why Electron, but that's a different question after all.
And all of that is completely out of your control and competence budget, unless you’re fine with shipping your first 50kb updates ten (metaphorical) years later.