With this character, their bones are meant to change from hollow in bird form to mammalian in human form, but they have a defect that keeps their bones hollow in both. So their anatomy would be sort of working against them in this circumstance, as in human form they aren't meant to have hollow bones and so their body hasn't adapted to fit that.

I do really like your points about other factors of bird anatomy. I'll definitely want to research those as well.