How did you measure the git's size? If you looked at the total size of the entire xonotic directory, you have to roughly divide this by two because git hosts the entire history of the project in its directories, and that includes a copy of the files that sit outside of git's own directories. So, my xonotic dir is 11GB, excluding the git directories brings that down to 6.1GB.
There's a huge gain from converting textures from .tga to .jpeg and/or .dds. Apparently, tga files alone total to 4.3GB of space.
I don't know if nexcompat is part of the release, but that's worth 500MB as well.
Here's some stats on filetypes on all default git directories in .xonotic (plus mediasource, hence the .rar files are project files in mediasource for example)
Code:
.tga    4393.3M
.zip    245.5M
.rar    201.9M
.wav    196.7M
.pk3    148.6M
.ogg    132.2M
.psd    118.7M
.jpg    95.2M
.png    63.9M
.7z     49.1M
.blend  48.4M
.map    38.3M
.md3    35.7M
.ttf    32.0M
.iqm    30.5M
.dll    26.8M
.ase    25.0M
.xcf    18.3M
.dpm    15.7M
.max    14.1M
.dylib  13.1M
Without mediasource:
Code:
.tga    4377.0M
.wav    191.7M
.ogg    132.2M
.pk3    113.2M
.jpg    94.8M
.png    46.7M
.map    38.3M
.md3    35.7M
.ttf    32.0M
.iqm    30.5M
.dll    26.8M
.ase    25.0M
.dpm    15.7M
.dylib  13.1M
Edit: for the record, I grabbed this awk script 
from stackexchange and modified it a bit. Dirty stuff ahead:
find .  \( -name '.git' -o -path './data.old' -o -path './mediasource' \) -prune -o -type f -printf "%f %s\n" | awk '{sub(/.*\./,"",$1);count[$1]+=$2} END{for (var in count) print "."var"\t" sprintf("%.1fM",count[var]/1024/1024);}' | sort -t$'\t' -k 2 -h -r
EDIT2: You don't need git for map creation. And in git, I'm guessing netradiant doesn't need anything outside of the map repo. Downloading that alone takes 5 GB of disk space in total.