Ruby big array and memory

I created a big array a, whose memory grew to ~500 MB:

a = []

t = Thread.new do 
  loop do
    sleep 1
    print "#{a.size} "
  end
end

5_000_000.times do
  a << [rand(36**10).to_s(36)]
end

puts "\n size is #{a.size}"
a = []

t.join

After that, I "cleared" a, but the allocated memory didn't change until I killed the process. Is there something special I need to do to remove all these data which were assigned to a from the memory?