Yugi was always small for his age. Even as a baby, he was small. At 4'2", Yugi is the smallest one of his friends. Tea, Tristen and Joey towered over Yugi. When Yugi turned into the Pharaog, he would grow slightly over 6 feet tall.
It was Tea who came up with the idea of unbirthing Yugi. First of all, Tea was almost 6'0" herself. She used to write stories about taking Yugi between her legs and into her womb. Plus, Tea had a crush on Yugi and unbirthing him seemed to be a turn-on. But Tea wasn't so sure that she could simply take Yugi up her birth canal. Plus, she wanted to experience what it felt like to have someone in her womb.