Adam Monsen

// config
var dbname = 'dev_bv';
var measure = 'count'; // or 'size'
var numTopCollections = 5;
 
function updateTopCollections(collection, stats, topCollections) {
    var thisCollectionObj = {
        'name' : collection,
        'count' : stats.count,
        'size' : stats.size
    };
    for(var i = 0; i < topCollections.length; i++){
        if (stats[measure] > topCollections[i][measure]) {
            topCollections.splice(i, 0, thisCollectionObj);
            break;
        }
    }
    if (topCollections.length < numTopCollections) {
        topCollections.push(thisCollectionObj);
    }
    if (topCollections.length > numTopCollections) {
        topCollections.pop();
    }
}
 
db = db.getSiblingDB(dbname);
var collections = db.getCollectionNames();
var topCollections = [];
 
for(var i = 0; i < collections.length; i++){
    if (collections[i].match(/^system/)) {
        continue;
    }
    var stats = eval('db.' + collections[i] + '.stats()');
    updateTopCollections(collections[i], stats, topCollections);
}
 
printjson (topCollections);

mongo --quiet topCollections.js