return large data by reference or as return in function?

On the job today I had an arguement with a collage about passing large data between scopes. The myth was that reference uses less memory/CPU usage when passing between 2 scopes. We build a proof of concept who was right... so:

function by_return($dummy=null) {
    $dummy = str_repeat("1",100 * 1024 * 1024);
    return $dummy;
}

function by_reference(&$dummy) {
    $dummy = null;
    $dummy = str_repeat("1",100 * 1024 * 1024);
}
echo memory_get_usage()."/".memory_get_peak_usage()."\n";
//1 always returns: 105493696/105496656
$nagid = by_return();
echo memory_get_usage()."/".memory_get_peak_usage()."\n";
unset($nagid);
//2 always returns:  105493696/210354184 even if we comment 1st part
by_reference($dummy);
echo memory_get_usage()."/".memory_get_peak_usage()."\n";
unset($dummy);

But it seems that by reference it consumes more memory according to function "memory_get_peak_usage()"

As you see, using large data for returning is much smarter than using as reference but the question is, why? Any enlightening is welcomed :)