I had a conversation with someone on the Release Team; we apparently don't remove enough packages post-install.
When a livefs is generated, it also generates a livecd.lubuntu.manifest-remove file. Here is an example:
For example, if btrfs filesystems aren't used, the package support shouldn't be installed.
Ubiquity goes through and removes unneeded packages if they're on that list. This is a bit tricky to implement because we need the correct logic. @wxl, could you take a look at the list and help me write some pseudocode for what we should remove here?