I don't have nginx installed (got the number from a web download of the .deb package), but running this for apache:

$ echo $((`ldd /usr/sbin/apache2 | cut -d">" -f 2 | sed "s/(.*$//;s/ //" | xargs du -L | cut -f 1 | sed "s/$/+/" | xargs echo` 0))

3200

So 3.2 MB of shared library dependencies. 1.8 MB just being the libc which is almost guaranteed to be used by a different program already.