Apple was a little more forthcoming in the list of improvements and fixes in macOS High Sierra 10.13.4, although it still doesn’t seem to account for a 2.3 GB update, even allowing for all the EFI firmware upgrades which came with it.

My list of components of macOS Sierra 10.12.6 which are included in 10.13.4 released on 29/30 March 2018 includes:

• Updated versions of App Store, Automator, Calculator, Calendar, Chess, Contacts, Dashboard, Dictionary, DVD Player, FaceTime, Font Book, iBooks, Image Capture, Launchpad, Mail, Maps, Messages, Mission Control, Notes, Photo Booth, Photos, Preview, QuickTime Player, Reminders, Safari (11.1), Siri, Stickies, System Preferences, TextEdit, and Time Machine.
• Updated versions of the utilities Activity Monitor, AirPort Utility, Audio MIDI Setup, Bluetooth File Exchange, Boot Camp Assistant, ColorSync Utility, Console, Digital Color Meter, Disk Utility (fewer errors seen), Grab, Grapher, Keychain Access, Migration Assistant, Script Editor, System Information, Terminal, and VoiceOver Utility.
• Updated Audio Plug-Ins.
• Updated fonts, including Charter, DIN Alternate Bold, DIN Condensed Bold, Gujarati Sangam MN, Rockwell, Apple Color Emoji, ArabicUIDisplay, ArabicUIText, GeezaPro, and the SFNSDisplay family.
• Updated Messages plugins for AIM, Bonjour and Jabber.
• Updated PDF services to save to iCloud Drive and to Web Receipts.
• Updated QuickTime codecs for Apple Intermediate and Apple MPEG2, and several other components.
• Updated Spotlight importers for iBooks Author, iWork and Microsoft Office.
• Several updated Widgets.
• Updated Accessibility support.
• Updates to many Automator support components.
• Most Core Services components updated.
• Most kernel extensions updated.
• APFS updated to version 748.51.0 (from 748.41.3 in 10.13.3), with new versions of all its tools.
• Updated most frameworks, public and private.
• Updated Perl 5.18
• Updated most Preference Panes.
• Updated many Spotlight importers.
• Updated TCL 8.5.
• In /bin, updated versions of launchctl and other tools.
• In /sbin, several updates including launchd, reboot and shutdown.
• Updated many tools in /usr/bin, /usr/sbin and /usr/libexec.

The bundled EFI firmware update should increment the reported Boot ROM version of every Mac, apart from those which have already undergone update during beta-testing of this update. I have updated my full listing of version numbers to take this into account.

Apple doesn’t explain what has changed in the EFI updates; from reports during 10.13.4 beta-testing, these should include further fixes for Meltdown/Spectre vulnerabilities.