And the smaller size is the federation's preferred dimensions. And the 90x60 doubles as the u12 11v11 dimensions for this year. Neat!
Regardless of the alterations CalSouth made to the national standards, any person watching any of those teams play could pretty easily see that the fields were too big. Thankfully, at this age, the teams that can actually play are starting to benefit from the width rather than be harmed.
As to whether it's a good idea to play on fields that big: it's not. The fact they could have lined the fields any way they wanted and went with the size they did demonstrates just how challenging it is to get people to do what's needed for development.