Mismatch of Download sizes in discover

I am a Fedora user. When I receive updates of number of packages. Example in total :

The discover says that,

It is 1.5 GB while as dnf says that it is 552 MB. Why is this ?

It fetches updates from rpm repos, why not fetch the sizes too. The same case applies for flatpak too.

The update will be about 15MB or so for FreeDesktop runtimes but it will show full size of the package as if I downloading for the first time.

Is this the same in debian based ones too ?