Yes, but that is a choice. It doesn't have to do that.

Because, instead of "trusting" the update (or rather codebase) of a messenger, we now have to trust the complete browser bundle.

They don't have to, but question was why it takes 100mb and you got the answer.

You may additionally ask why Electron, but that's a different question after all.

That’s only formally a choice. In reality, you’ll depend on some delivery system (among many other systems) that is a part of some build/dev system that is a part of a job market conjuncture.

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.