^That's a circular argument.
Not really. They wouldn't design special cargo containers for just one ship class. They would use the standard DY series cargo containers, the ones that were designed to go around the cylindrical hull of the DY's. This hull design simply doesn't permit such a placement with said containers. So, they loose some theoretical cargo capacity, but they can charge a premium for shipping because of it's reduced transit time, which offsets the smaller total amount of cargo shipped.
Sounds perfectly logical from a business standpoint.