Layer mash is finely milled feed that when mixed with water makes a "mash" consistency. It is meant to be fed moist, which is why it's ground fine. Not many folks feed it moist nowadays but it traditionally was fed as a moistened feed. The mash I currently have access to at this local mill is more coarse ground than I am used to buying and it seems to have more corn in it than the feed I used to get at my old town.
As with anything that is bagged, the more fine the particles, the more actual volume you receive per bag. Like pecans....it may say 12 oz on the bag but if it's finely chopped it has more total volume of nuts in the bag than do the bags with pieces or whole nuts. With fresh milled feeds, the bags are all standard size and the boys filling and tying those bags just fill it, tie it closed and put it on a pile. Bigger pieces of grain will have spaces in between and won't fill the bag as solidly and fully as will finely ground grain, which is why, if I'm going to buy premixed feed, I always buy mash instead of crumble or pelleted style feeds....more for your money.
When one purchases a product by weight, one recieves the stated weight. ie. 12 oz will contain 12 oz of product. When one buys finely chopped product vs. larger pieces or whole, one will actually recieve LESS volume of that product if finely chopped since the fines will occupy air space that is created if it was whole. When the guys bag the grain/ feed at the mills ... there is a device at the dispencer that measures the WEIGHT of the grain/ feed that fills the bags that shuts the grain/ feed flow AUTOMATICALLY when the desired wieght ( example: 50 lbs.) is reached. Therefore, the finely ground grains will have less volume than the whole grains.( I know a mill owner personally that I buy my grains from. I help the guys grind and bag one ton of fresh ground grains that I buy every 2 weeks for my horses. Yes, each and every bag must be labeled as to contents and weight by law.) If the mill owners allowed their employees to fill all the bags equally by volume, no matter if the grain was whole or finely ground, , they would be out of business in short order. Besides, when the label states a weight or volume , it had better be that stated weight or volume or the BIG brother from the Dept. of Weights and Measures will decend upon them with HUGE fines for deceptive labeling/ business practices.