You're not supposed to be able to modify it in the first place, but some scenes don't check for it being a real peen when applying tfs. So it resets whenever it's flipped off and on.
Getting some kinky mage to tune ours and/or Cait's ghostlight up for different sizes and basic shapes would be a neat option at some point, if the way ghost dicks are currently implemented won't make coding such content too much of a hassle.
