If the pymongo code considers a document to be too large then it raises:

InvalidDocument: document too large - BSON documents are limited to 4 MB

Please change the message to include the size of the document itself. For example if my doc is 300MB then I have seriously screwed up whereas if it is 4.1MB then a little fine tuning or using Mongo 1.7 will fix the problem due to 8MB limit.

Note that the message appears once in Python code and twice in C code.