mongodb_collection_sizes.js

var collectionNames = db.getCollectionNames(), stats = [];
collectionNames.forEach(function (n) { stats.push(db[n].stats()); });
stats = stats.sort(function(a, b) { return b['size'] - a['size']; });
for (var c in stats) { print(
stats[c]['ns']             + ":  " + 
"total_size=" + Math.floor(stats[c]['size']/1048576) + " mb,  " + 
"idx_size=" + Math.floor(stats[c]['totalIndexSize']/1048576) + " mb,  " +
"allocated=" + Math.floor(stats[c]['storageSize']/1048576)   + " mb,  " +
"num_docs=" + stats[c]['count']
);}