Having written a fair amount of SPA and similar I can confirm that it is actually possible to just write some JavaScript that does fairly complicated jobs without the whole thing ballooning into the MB space. I should say that I could write a fairly feature-rich chat-app in say 500 kB of JS, then minified and compressed it would be more like 50 kB on the wire.

How my "colleagues" manage to get to 20 MB is a bit of mystery.