ffmpeg source is 405 MB whereas fedora ffmpeg src is only 6.4 M

[[Unknown]]

The repo you are looking at includes precompiled binaries for supported platforms, not just sources. It also includes some version history metadata (from git) which you are not accounting for in the rpm. They are completely different numbers.

If you need to redistribute Linux-only source archives, delete the iOS/Windows/Symbian/etc. .a/.lib files and such.

-[Unknown]