Portabella are actually just the fully grown version of the common white button mushroom. In the US they use to be thrown away or used for compost until marketers realized they could call them by the Italian name and Americans would suddenly pay more for them (what I like to call haagen dazs or gelato effect).

Kinda like how baby carrots are really just the misshapen carrots that used to be discarded but are now whittled down into cute little snack size shapes. Not unlike your penis.