Where to get the full offline-capable installer for macOS Mojave (.1 update)?
The 22 MB "stub" installer is downloaded when your Mac is having trouble getting the correct data from Apple's software update catalogs. This is quote common during the first 24 hours or so, as it takes a little while for the updates to propagate across the CDNs. The solution in this case is to delete the stub, wait a couple of hours, and then try again.

There is also a useful script by Greg Neagle that lets you inspect which installers are available in your update catalogs, and can download them for you: installinstallmacos.py

PS: If you have a Mac managed by an organization, you may also get the stub installer if your administrator hasn't approved the new update, e.g. with Reposado.