On my system, it's about 4MB output (>87.000 lines).
Found out using
apt-cache unmet | wc -c for number of bytes and
apt-cache unmet | wc -l for number of lines.
I did not learn much from having a quick glance at the source code, but I strongly guess this command is designed to print all unmet dependencies of all installable packages, instead of only those packages which are actually installed.
man apt-cache is in my opinion not unambiguously worded, but here are the important snippets:
unmet displays a summary of all unmet dependencies in the package cache.
Print only important dependencies; for use with unmet and depends. Causes only Depends and Pre-Depends relations to
be printed. Configuration Item: APT::Cache::Important.
So you can also narrow down the output somehow if you add the
-i argument. I am not sure what exactly this filters out, but it results in less than about 10%, roughly 7.000 lines in my case.
$ apt-cache unmet -i | wc -l