some people create their index thinking about the usual query more than inserts.
For example for gridfs, people usually will query db.files.find(

{filename: "file"}

).sort(

{uploadDate: -1}

) to have most recent first.
And so index may get created like

{filename: 1, uploadDate: -1}

(which is what Python driver does).
But the actual inserts go with increasing date, so the btree grows by splitting 50/50 (left side).
The index ends up being twice as large as if the index was created in reverse order.

Inserted 100000 increasing numbers into ascending index:
> db.time.$time_1.stats()
{
"ns" : "mydb.time.$time_1",
"sharded" : false,
"primary" : "shard0000",
"ns" : "mydb.time.$time_1",
"count" : 452,
"size" : 3702784,
"avgObjSize" : 8192,
"storageSize" : 13191168,
"numExtents" : 4,
"nindexes" : 0,
"lastExtentSize" : 10438656,
"paddingFactor" : 1,
"flags" : 0,
"totalIndexSize" : 0,
"indexSizes" : {

},
"ok" : 1
}

Inserted 100000 decreasing numbers into ascending index:
> db.time.$time_1.stats()
{
"ns" : "test.time.$time_1",
"sharded" : false,
"primary" : "shard0001",
"ns" : "test.time.$time_1",
"count" : 813,
"size" : 6660096,
"avgObjSize" : 8192,
"storageSize" : 11141120,
"numExtents" : 4,
"nindexes" : 0,
"lastExtentSize" : 8388608,
"paddingFactor" : 1,
"flags" : 0,
"totalIndexSize" : 0,
"indexSizes" : {

},
"ok" : 1
}

we should split 90/10 on left of leftmost so that the order of index really doesnt matter.