Re: the ...-unload-hook convention doesn't work

[Richard Stallman]

    Essentially: how do I find out what is taking up all memory in Emacs?
    Perhaps a package that report these things would be handy.

I wish I knew.  However, I am surprised your mapatoms runs out
of space.  It looks like all it will do is add a line
to the buffer for a fairly small subset of symbols.

It would be interesting to debug were most of the consing goes
inside that mapatoms.  Maybe running that code with a breakpoint
in malloc would show you where.

It might be due to undo entries.