If I save the *compilation* buffer and restart emacs, memory usage goes up
modestly when I load the file (52MB; it is automatically fontified by emacs
when I load it):

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
peter    19037  0.6  0.7 441776 115476 ?       Ssl  11:59   2:49 emacs
--daemon
peter    19037  0.6  1.0 495832 169776 ?       Ssl  11:59   2:50 emacs
--daemon

and memory usage returns to about the previous value when I kill the buffer
(I don't need to run 'garbage-collect).

So, the problems seems to be somewhere in the "compile" command:
(a) it uses a *lot* more memory than needed to display the result
(b) it doesn't free that memory when compilation ends or when the
compilation buffer is killed

My "compilation" is actually compile plus 8200 tests, speeding them up by
using GNU-parallel.