Why does Chalav Yisrael have any spoilage issues?
Bacteria.
Personally, I buy smaller amounts more frequently during the summer, and very infrequently have spoilage. The point about gallon containers is valid, though. For some reason, probably handling, it has been observed that CY milk has more issues than CS. I don’t know if anyone ever did a real head to head comparison or it’s just a stereotype, but there is that perception. A gallon container will be more likely to spoil than one half gallon container bought after another, and even than two bought together, because it will spend more time unrefrigerated.