Is there an El Capitan Update only? The title is a bit confusing so I will explain below:

The El Capitan downloaded from the Apple Mac Store is big enough to do a fresh installation as well as an upgrade to your current OS version.

The size of this however is very large.

My Question

Is it possible to download just the update from OS X 10.10.5. For example to update from 10.10.4 to 10.10.5 there is an updater which is smaller than the combo update. Is there one for an update to 10.11.0/1.

Hope this makes sense
No, there isn't. El Capitan is a major release which replaces a lot of (or even most) files of the OS. So the "big" Installer from the Store is both an Installer and an Updater.