The quality of one item seems to be higher. It likely takes more ingredients in order to simulate the effort and expense a crafter would need to put into an item of greater quality. That's the in game lore reason. The out of lore dev reasoning would be that the size of an item in game has some relevance in the number of materials used, but it's just a guideline. There are thousands of items. If they had to adhere to strict rules of size = mats used, it would be impossible as hundreds of furnishings are added every year.