Distribution transformers are not usually thought of as having a winter rating. On the other hand, distributions lines definitely do. Sometimes work on lines can only be done during the winter, because the load has to be shifted to lines that can only handle the increased temporary load during the winter.

There is a science and an art to sizing residential xfmrs. Generally is depends on the square footage, number of bedrooms and type of fuel used to condition the house.

Probably where GP is running into problem(like most electrical utilities) is that the use of electricity per square foot keeps going up.(caused by the increase of electrical devices in the home) So while the xfmr might have been sized correctly when installed 20-40 years ago, it is no longer sufficient. This causes problems.

Source: I was a distribution engineer for 7 years.