请问a type of small bag中的bag为什么不可数?

请问a type of small bag中的bag为什么不可数? Mary would like a small bag of fruit.对a small bag提问 a small red bag 还是 a red small bag? There is a small bag of salt on the table. A bag of b_____. a bag of sweets a bag of ( ) A Bag of Kindness a bag of rice a bag of wind a bag还是an bag?为什么? small bag of clothing for carrying