I think the OP meant huge in terms of complexity rather than memory usage..
Brave is a web browser, so I'd give electron most of the credit for making that work. Popcorn time I don't know anything about.
Slack and atom though? Both are simple client UI's and both have much less bloated alternatives.
Yes, admittedly, none of these are that complex. I thought Visual Studio Code was built ontop of Atom, but I might be wrong. Both built on Electron anyway - which was originally part of Atom.