As you can see from
$ du -sch /usr/local/texlive/2012/texmf-dist/* | sort -hr
3,8G insgesamt
1,5G /usr/local/texlive/2012/texmf-dist/doc
1,4G /usr/local/texlive/2012/texmf-dist/fonts
726M /usr/local/texlive/2012/texmf-dist/source
218M /usr/local/texlive/2012/texmf-dist/tex
22M /usr/local/texlive/2012/texmf-dist/tex4ht
16M /usr/local/texlive/2012/texmf-dist/bibtex
11M /usr/local/texlive/2012/texmf-dist/scripts
6,6M /usr/local/texlive/2012/texmf-dist/dvips
3,9M /usr/local/texlive/2012/texmf-dist/metapost
2,1M /usr/local/texlive/2012/texmf-dist/omega
1,9M /usr/local/texlive/2012/texmf-dist/ls-R
1008K /usr/local/texlive/2012/texmf-dist/context
444K /usr/local/texlive/2012/texmf-dist/metafont
308K /usr/local/texlive/2012/texmf-dist/makeindex
256K /usr/local/texlive/2012/texmf-dist/pbibtex
48K /usr/local/texlive/2012/texmf-dist/texdoctk
36K /usr/local/texlive/2012/texmf-dist/mft
20K /usr/local/texlive/2012/texmf-dist/texdoc
12K /usr/local/texlive/2012/texmf-dist/web2c
the major part is documentation and fonts. If you are satisfied with a single font and without documentation, the distribution can be much smaller. There were several attempts to create such a minimalistic distribution, but I'm not sure if they are still available (edit: I just rediscovered one: grailtex).
And as daleif stated in his comment, you can install a much smaller TeX Live as well.
As real overhead, maybe the source
directory counts, which is also quite large. It consists of sources of most of the stuff in the other directories such as LaTeX packages etc. But this is rather a political than a technical question, therefore I leave it to the TeX Live people to answer it (IIRC they already answered it, maybe on the TeX Live mailing list).
lualatex
executable (and necessary supporting programs), a minimal amount of packages and use system fonts you'll probably only need a couple of megabytes. But that won't be "TeX Live" anymore. Also your TeX-in-browser idea will very likely need a special implementation anyway. The standard TeX engine can't just be plugged into a browser rendering engine.300MB
. If you strip away the MkII stuff (including binaries), then the size will be reduced by more than half.