I currently use OS X 10.9.1. The size of the OS X 10.9.2 update from App Store is ~460MB, while it is ~869MB (delta or combo) from the site. It matters to me because I have to install it on various machines and I am on a limited bandwidth, limited speed internet connection.
The so-called combo update combines (hence the name) the updates for all possible models of Mac's and all possible combinations of partial updates that may have been released between 10.9.1 and 10.9.2.
As such it can be (and should be) used to update any arbitrary Mac.
The AppStore download is customized specific to your Mac and it's current update status.
It is not suited for any other machine as those machines are most likely not 100% identical to your Mac.
Because it is tailor-made for your Mac it doesn't contain any extra's. That makes it smaller, but you will need those extra's for the other machines.
So you will have to download the combo-update.
This you only need to download once and you can then run it on each machine.