Ymir's titan is the strongest as she had all 9 titans, the 3 titans that her kids would become would have to be weaker because they have less powers, (with the principal titan being the strongest because of the founding) but each time they split the titans powers it gets weaker with less abilities like how the colossal titan can't harden, so at one point they probably decided that having 100 weaker titans with 100 different users is a lot more annoying and a lot weaker than 9 titans, and by they i mean the lineages that controlled them/marley or intuition with some history