As per a post on the NEOgaf forums the Xbox One digital download version of Zoo Tycoon is 2.6 Gb. This is quite small compared to all the other launch titles and a lot more reasonable. Probably the first thing I purchase since everything else is humongous.
source: http://www.neogaf.com/forum/showpost.php?p=89132917&postcount=154
source: http://www.neogaf.com/forum/showpost.php?p=89132917&postcount=154