memory usage

Sorry this may be quite basic but just wanted to confirm it:
Let's say that I have a proc which generates a large dict and returns it. Then when used, the caller assigns it to a variable.
How much memory is used, 10mb or 20mb? (let's ignore any extra memory needed to manage the dict structure and the proc calls).
proc generate_10mb_dict {a b c} {
  # generate a large dict, in d
  return $d
}
set my_data [generate_10mb_dict $a $b $c]
what is the ram use at this point?