We use mongo 2.2.0, replica set of 3 instances, no sharding.
Seems, 44 times growth definitely looks like a bug, not a feature.
Collection is used as a fast cache, so it is write/remove intensive.
After we experienced this problem, we started to use power of 2 sizes. But it didn't help.

db.stats(1024*1024*1024);
{
"db" : "image-url",
"collections" : 10,
"objects" : 3324847,
"avgObjSize" : 401.2754499680737,
"dataSize" : 1,
"storageSize" : 44,
"numExtents" : 64,
"indexes" : 12,
"indexSize" : 0,
"fileSize" : 51,
"nsSizeMB" : 16,
"ok" : 1
}