Backward compat has nothing to do with the size of the stdlib, AFAIK. It seems you want to pick on a different part of the language ecosystem.
It's true this a matter of taste, but also worth noting that the OCaml compiler devs have made it very clear they are open to well-motivated extensions of the stdlib, and it has been growing at a decent clip in the last few years.
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.)
It's true this a matter of taste, but also worth noting that the OCaml compiler devs have made it very clear they are open to well-motivated extensions of the stdlib, and it has been growing at a decent clip in the last few years.