I learned about this in child psychology years ago at RIT.
That's part of the reason why it is discouraged to give broken crackers or cookies to toddlers in day cares.
If you put a broken cookie (broken into 2 pieces) on one plate and a whole unbroken cookie on another plate for 2 different toddlers, the one with the whole piece would likely complain that the one with the broken pieces has more cookies than him/her.
Some little boys like to feel like they're older when they eat as much as their father. So, breaking a cookie into 2 pieces may make the little boy feel like he's eating as much as his father when the father is eating 2 whole cookies.
The same applies to drinks. If you had a wide cup filled to the same height as the drink in the narrow cup, both would likely be perceived as the same to the toddler.