Rbkit shows the RES mem size grows, but top shows RES keeps flat

[dzl84]

I use rbkit to profile the following code. It runs with ruby 2.2.0. After running for 1min, rbkit shows RES size is ~100MB, while top shows the RES is about 13MB.

require 'rbkit'
require 'timeout'
Rbkit.start_server

100000.times do
timeout(3) {
puts 'hello mem!'.freeze
sleep(2)
}
end

So what is the RES size in rbkit? How can I get the same value of top result?