require "benchmark/memory"

class A
  def self.class_method
    puts 'hello'
  end
  def instance_method
    puts 'hello'
  end
end

Benchmark.memory do |x|
  x.report("class_method") { A.class_method }
  x.report("instance_method") { A.new.instance_method }

  x.compare!
end

#Calculating -------------------------------------
#hello
#        class_method    40.000  memsize (     0.000  retained)
#                         1.000  objects (     0.000  retained)
#                         1.000  strings (     0.000  retained)
#hello
#     instance_method    80.000  memsize (     0.000  retained)
#                         2.000  objects (     0.000  retained)
#                         1.000  strings (     0.000  retained)
#
#Comparison:
#        class_method:         40 allocated
#     instance_method:         80 allocated - 2.00x more