This is Zimbra's standard method for distribution. Each update is compiled seperately, patches are not normally used. I'd assume this has to do with the number of items normally fixed and also then they do not have to maintain patches for all the versions. It's easier to maintain a complete binary. Even on a 1.5 mb/s connection, 450 megs is a quick download from their servers.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now