I think the concern OP has is why a lot of the updates are so large. I use signal desktop and the UI hasn't changed in years. It begs the question what those 100mb are and whether it's actually necessary.
Signal desktop doesn't use incremental updates. Each "update" is just reinstalling the whole package. That's what those 100 MB are.
It's possible to make incremental updates with binary patches, but it's more difficult. I guess Signal have other priorities.
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.