We size bags based on the size of the opening, and there is a HUGE jump in dimensions between medium and large. Have you noticed? The opening on medium bags is 30 cm (12 inches) and the opening on large bags is 38 cm (15 inches). So when the opening on a large -- or even extra large -- bag doesn't meet the minimum dimensions, the bag won't be sold as that size.
That's where Medium-Deep bags come in. They are essentially large, or even XL bags, that have an opening narrower than 38 cm (15 inches). Medium-Deep bags are humongous bags, and a fantastic deal for you.
Here's an example: a large bag tapers to a narrower opening that is 36 cm (14") wide. That bag will not be sold as a large because it doesn't meet the minimum, so it's sold as a medium-deep. It's essentially a large bag that is sold at a substantial discount!
Minimum Dimensions on Medium-Deep Bags
30 x 28 cm (12 x 11")