With some diapers, the capacity is identical because the padding is identical. Those just make the shell larger so the front/back are longer and the wings are wider, but the padding remains the same size and shape. They tend to ship in the same count per bag/case and similar price since the unpadded plastic doesn't affect the total product size much.
Others, the larger diapers have more padding in them. Those tend to ship in different quantities between sizes, and costs per piece.
Between brands and models, it's hard to compare. Even a diaper that's thicker may have a lower capacity than a thinner one. Pulp is cheap, and adds bulk, and doesn't change much when it gets wet, but it doesn't hold well. Sap is added to greatly improve capacity, and causes diapers to swell when wet, and locks in wetness better, but lowers wicking performance and can greatly affect price. So there's a lot of variation there.