The above are two dumplings. The size differs because they have different fillings. One is filled with slices of chicken, egg and other stuffs while the other is filled with char siew or minced pork with sweetish sauce.
My son love char siew pau and he was amused to see the two paus in different sizes. He called it mama pau and baby pau. When we asked him what he wants, he will say baby pau. So, we know he means char siew.