Can anyone explain these file sizes?

[Bojevus]

Why in the world would the monthly frontier pass update that adds a couple civs be a 12.9 gigabyte download?

 

[Voenkomat]

It's like call of duty.The game itself weighs 200 gigabytes and updates weigh about the same.

 

[fotonut]

Hmm, the download was like 300MB for me (don’t remember exactly but nowhere near 12GB).