The size of the stdlib has a
huge impact on backwards compatibility and how much pain is caused by maintaining said compatibility
for the authors of the stdlib. I mean... it's hard enough for just any regular library to do that sensibly. So much so that semver (misguided as it is) was invented.
IIRC Ocaml always compiles everything from source which has its advantages, but even that is a can of worms. (And sometimes binary compatibility can actually be easier, see e.g. Scala.)