Why MongoDB starts to split before 64MB
The mongos processes control when automatic splitting happens (you can also pre-split, and split manually).  The heuristic they use is a bit more complicated than I am about to describe, but you can use it as a rough guide:


Each mongos keeps track of data written to a particular chunk
At ~20% of the max chunk size (default is 64MB), it will try an autosplit
If valid split points are found and returned (by splitVector), it will split
If not, it will try again at 40% (+20%)


Hence if you have inserted enough data for valid split points to be found, you will see splitting before the 32MB mark (number of documents are a valid reason to split, it's not all size related).

If you do not wish for this to happen, you can start the mongos you will be using to write data with the --noAutoSplit option or you could run with a config server down (which means all splits will fail when attempted because the meta data is read only).  Generally, that would not be recommended for production, but can sometimes be needed for testing purposes.