Mongoid.default_session.collections.map do |c|
s = Mongoid.default_session.command(collstats: c.name)
[c.name, s["size"] + s["totalIndexSize"] ]
end.sort_by { |n, s| -s }.take(10).each do |n, s|
puts "#{n}: #{s}"
end; nil
Outputs something like this:
squirrels: 5432044544
monkeys: 949976816
sleuths: 723951712