The logic we use right now on mongos to determine whether adding a write to a batch would make a batch too big only takes into account the total estimated size of each individual write operation so far. We do not appear to take into account the size of top-level command fields, for example let on an update command, but we likely should.

Relevant logic:

• Calculate the size of the individual write op
• Check if that calculated size + existing ops calculated size pushes us over the MaxBSONObjSize
• Increase our size estimate each time we add a write

We are going to factor in such fields for the bulkWrite command specifically in SERVER-73536 but it seems we might want to make similar changes for insert/update/delete as well.

SERVER-74806 was a recent case where $merge generated a too-large internal update command because its similar logic did not factor in let and legacy runtime constants.

This is somewhat related to SERVER-53387 though that is about factoring in metadata size.