No deberían pesar más de 16mb ya que daría error al insertarlos/updatearlos.
Muy mal tiene que estar el diseño de esa db para que ocupen tanto los docs.
Quote del mismo mongo boss :
So, on your blog example, 4MB is actually a whole lot.. For example, the full uncompresses text of "War of the Worlds" is only 364k (html): http://www.gutenberg.org/etext/36
If your blog post is that long with that many comments, I for one am not going to read it :)
For trackbacks, if you dedicated 1MB to them, you could easily have more than 10k (probably closer to 20k)
So except for truly bizarre situations, it'll work great. And in the exception case or spam, I really don't think you'd want a 20mb object anyway. I think capping trackbacks as 15k or so makes a lot of sense no matter what for performance. Or at least special casing if it ever happens.
-Eliot
No tiene sentido tener objetos tan grandes, reventarías la ram en cuanto hubiera un de operaciones.
Pero si aún así quieres hacerlo usa como dices el gridfs, : https://docs.mongodb.com/manual/core/gridfs/
Partes en trozos el doc.
Saludos