The USPS is bleeding money at a rate of $25 million dollars a day.
The more weight AND the more cubic volume of a shipping package cost more to transport. The have two options here. The can raise prices drastically, or then can reduce the volume a container can hold, and increase prices slightly.
It isn't going to get any better, either. Suffice to say that right now the cost of shipping is as low as it's every going to be, and the "Priority Mail Large boxes' are as big as they will ever be.
Losing $25 million a day is pretty serious business. That's nearly $10 Billion a year in losses. You have to expect things to change for the USPS worker, retirees, and for their customers. Smaller boxes and higher shipping rates ARE in our future. So is longer transit times, fewer services (ie: Saturday Delivery), and closure of Post Offices.
Without pointing fingers, we will all have to get used to it, it's the new reality.