If a user is wearing the Classic ROBLOX Pumpkin Head hat and moves the Proportions slider in the mobile Avatar Editor, the hat will vertically grow. This only occurs when the Body Type slider is at a position other than 0%. This happens 100% of the time.
Repro: in the avatar editor on the Roblox mobile app, equip the Classic ROBLOX Pumpkin Head hat and set the Body Type slider to 100%. Move the proportions slider left and right and you will see the hat grow!
I have only tested this on my Xiaomi Mi A1 (running Android 8.1), although I’m assuming this occurs on all mobile and tablet devices. It doesn’t apply to the normal Roblox website avatar editor.