I've tried to install security update 2019-002 on my HighSierra in 3 different ways. Through the App Store, the terminal (softwareupdate) and the file I've downloaded from Apple's site. The result is always the same. I restart, it starts installing and then I get the message "macOS could not be installed on your computer. The target disk is too small for this operation- Quit the installer to restart your computer and try again."
The thing is, I have plenty of space available. The update requires 5.31GB of space and my disk has 36.68GB available: