It probably isn't really using that much but it will have reserved that much. V8 reserves memory in chunks of 256MB using virtualalloc with MEM_RESERVE on Windows (I think). It has to do that for each isolated process where an isolate is a tab, webview, worker, etc. A page that's built up from 4 isolated processes will reserve 1GB of RAM. That's only virtual alloc though, so other OS processes can take the memory if they really need it.

There is essentially no way to tell if a JS app is using a lot of memory just by looking at what the process has reserved. There's loads of things that end up in that space - cached pages, cached compiled code, cached bitmaps of rendered pages, etc.

The task monitor tells you what Chrome or Chromium (e.g. Electron) is doing, not what the web app is doing.

There is a good argument to suggest Chromium is hogging more than it should. That's not really WhatsApp's fault though.