No idea. It has changed a bit..

"max_response_len": 190c6f39c983ee8e61c73d20ad771ae46d8fdf16
max_doc_size: fb2b9c7a5fa1816be3c90ac7a2ae94ed0e6131ad
"large bindata": 02cb7bb4d71c6cf9ebf6c841cd2d95f20c30d050

I suppose it is just a quick sanitycheck..
As long as it is not used for checking the full message size (which is determined by ismaster() response), or the document size, - only individual fields withing a document I suppose it doesn't matter the exact number..