Let's say there's 1,024 teams. Which there aren't, but let's imagine..

And let's say each team has 50 players.. Which it doesn't, but let's go crazy, here.

Each player has a name (128 bytes?) and then... say... 256 x 32-bit integer stats (256*4 bytes)
256 x 32-bit integers per player should be more than enough for incredibly detailed performance stats, as well as settings for "Create a character" type facial/body type stuff.

So, we're asking for 1024 bytes + 128 bytes for the name, which comes to 1,152 bytes.
(Note, although player performance stats might change over time, typically players don't change their names or facial features during the course of a season, but.. Let's assume those are in the save file, anyway, because the developers can't be bothered to check those stats which should already be in the ROM file!!)

1,152 bytes * 50 players = 57,600 bytes per team.
Multiply that by 1,024 teams = 58,982,400 bytes, or 56.25 Megabytes

Where the hell are they getting 5Gb save files from!?!?!?