In my environment, running the example above with the default dev profile I get
-rw-r--r-- 1 mseri staff 2534414 Nov 29 16:05 _build/default/foo.bc.js
While with --profile=release I get
-rw-r--r-- 1 mseri staff 12918 Nov 29 16:05 _build/default/foo.bc.js
Which are consistent with the numbers above.
EDIT:
I have also tried it on a beefier example: the js_of_ocaml demo using owl_ode.
In that case I get the following
-rw-r--r-- 1 mseri staff 6218237 Nov 29 16:13 _build/default/teaching_material/preypredator.bc.js
-rw-r--r-- 1 mseri staff 92098 Nov 29 16:14 _build/default/teaching_material/preypredator.bc.js