The problem isn't the size, the problem is that it adds features which are outside the scope of 'a Browser'.
The comments in this thread are going in loops.
"Hello shouldn't be bundled with a browser."
"But it is just a small wrapper around WebRTC."
"Yeah, but it's out of scope for the browser."
"WebRTC is a web-standard"If you take that into context when you look at Hello, you can see why the organization might want to leverage the Firefox user base to introduce something with negligible overhead (Hello) that would help to advance that mission.