I think transformers have gotten larger and heavier since most states have adopted some form of the International Energy Code, or something similar.
There is a paragraph in there about transformers having to meet some standard for energy efficency.
It is a federal law.
Public Law 109-58 of the Energy Policy Act of 2005 which went into effect in Jan 2007.
Transformers that meet these criteria are called TP-1 design, after the NEMA guidelines, are designed to be most efficient when based on a 24hr average loading of only 35%.