The full download would be GB not MB.
You can check your applications folder "Install macOS Catalina.app" .
If it is not some ~8.13 GB in size, delete any partial download and empty the trash. Try again.
https://www.apple.com/macos/catalina/
macOS Catalina - Technical Specifications - Apple Support
https://support.apple.com/kb/SP803?locale=en_US
System requirements macOS 10.15 Catalina
macOS Catalina will run on all Macintosh computers that supported Mojave:[1]
MacBook: Early 2015 or newer
MacBook Air: Mid 2012 or newer
MacBook Pro: Mid 2012 or newer
Mac Mini: Late 2012 or newer
iMac: Late 2012 or newer
iMac Pro
Mac Pro: Late 2013 or newer; Mid 2010 or Mid 2012 if upgraded with a recommended Metal-capable GPU