test.rb

def recurse(i)
  recurse(i-1) if i > 0
end
threads = ARGV[0].to_i
puts "Running with #{threads} threads!"
threads.times do
  Thread.new do
    recurse(500)
    sleep
    puts "done"
  end
end

require 'memory_profiler'
MemoryProfiler.report do
  Thread.new{sleep 0.1}
end.pretty_print

sleep 1
puts "#{Thread.list.count} threads running!"
sleep

allocated memory by class

1049568 Thread

sam              18582   0.0  0.1  2665088  12896 s002  S+    8:17am   0:00.09 ruby test.rb 100
sam              18568   0.0  0.2  3504496  35884 s001  S+    8:17am   0:00.14 ruby test.rb 500

VSZ is is allocated, but only turns to RSS when used, you can see the impact via playing with recurse amount or setting stuff on thread storage.

So RSS impact depends on call stack depth and other stuff that is stored on the thread stack (local rvalues like numbers and params, thread local storage and so on)