Actually the difference between a small cargo bay, a medium cargo bay, and a large cargo bay would be staggering...
A small cargo bay still needs a
1. Cargo handling system(to you know, put cargo in and out)
2. Storage systems (to hold it in place)
3. Climate control
4. Walls (for reference, shipping container has 20cm wall thickness, 15% of its diameter)
So for small bays most of your volume is not spent actual storing cargo...