[fwn]

WhatsApp reduces the quality of your media and saves it in a second folder of your gallery.

If you forward already converted media it will be taken from the gallery in it's compressed, substantially smaller version.

[oarsinsync]

On iPhones it has a splash screen for compression and then a separate spinner in the chat for the upload progress for videos. That second upload spinner doesn’t occur for immediately forwarded videos.

[fwn]

That is interesting and different from what I have experienced on Android. I've no explanation for that.