I was curious to know the difference between the two packages so wrote this quick query to see what packages they pull in. This command will show the packages that kubuntu-full depends on that kubuntu-desktop does not.

comm -23 

    <(apt-cache depends kubuntu-full | awk '/Depends/ {print $2}' | sort -u) 

    <(apt-cache depends kubuntu-desktop | awk '/Depends/ {print $2}' | sort -u)

The result at the time of writing (on 12.10) is:

anthy
calligra
calligra-dev
dictd
dict-foldoc
dict-gcide
dict-jargon
dict-moby-thesaurus
dict-vera
enscript
eyed3
finger
flac
kdeaccessibility
kdeadmin
kde-baseapps
kdeedu
kdegames
kdegames-card-data-extra
kdegraphics
kde-icons-mono
kdemultimedia
kdenetwork
kdepim
kde-runtime
kdesdk
kdeutils
kde-wallpapers
kde-workspace-data-extras
kinfocenter
konq-plugins
kubuntu-desktop
libqscintilla2-designer
libqt4-sql-odbc
libqt4-sql-psql
libqt4-sql-sqlite
plasma-runners-addons
plasma-widget-ktorrent
plasma-widget-lancelot
pyqt4-dev-tools
python-qscintilla2
qt4-demos
qt4-designer
qt4-dev-tools
qt4-doc-html
scribus
unclutter
vorbisgain
xnest
xrestop