If you equip the Tiny Potion (You will become tiny ofcourse).
Then if you equip the Small Potion (It will change the size to the Small Potion as it applies the last equiped potion).
If you then unequip the Tiny Potion (You will become normal size even if the Small Potion is still equiped).
The trick of summoning this is to unequip the last equiped potion while leaving the other equiped potions active.
Quick fix for this (Allow only one potion to be equiped at a time).