When something complex exists it’s usually because alternatives are worse. Would you have less issues with 10MB updates? 1MB? One megabyte is a lot of text, a good novel for a week of evening reading can be less than that.
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.
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.