Image compression will create a loss of image quality, hence why it is not used.
The android .apk does use actually image compression. The size goes from 2.5gb to 1.6gb. Anymore and images will start to lose quality.
Other sites will break your DL and waste your data. Mega is garbage, and racaty takes forever (hours) to DL even on high speed.
Chances are your data limit is already reached on Megas trashy shit because mobile networks by default use a router and assign a local IP. Unfortunately, Mega counts the router when assigning data limits and thousands and thousands of people are on that same public IP. Mega also forces you to download an app and make an account. Mega is just total shit.