Memory bloat when creating many new objects

When I run this and then watch the memory consumption of my ruby process in OSX Activity Monitor, the memory increases at about 3 MB/s.

If I remove the transaction it about halves the memory consumption but still, the memory footprint keeps going up. I have an issue on my production app where Heroku kills the process because of its memory consumption.

Is there a way of doing the below, in a way that won't increase memory? If I comment out the .save line then it's okay but of course this isn't a solution.

ActiveRecord::Base.transaction do
  10000000.times do |time|
    puts "---- #{time} ----"
    a = Activity.new(:name => "#{time} Activity")
    a.save!(:validate => false)
    a = nil
  end
end

I am running this using delayed_job.