We had a Herfeord heifer that when she calved had no udder at all to speak off. Her first calf would nurse adnd there would be milk dripping and pouring all over the place. Calf only nursed as long as any of the others did and weaned at just shy of 600 pounds. The next year the same cows udder looked like an old wore out set of glaoves, no milk at all. That claf weaned at well over 700 pounds. Her udder looked exactly the same after she was nursed then before except it was wet and slobbery after nursing. At the dairy there was one cow that came in and I asked if she was ready to be dried off since she didn;t look to have any milk. HThe look of her udder never changed much from start to finish, she's milking over 80 pounds a day.
I asked the vet about this phenomonon once and he just laughed and said he's never figured out where some of these cows store the stuff.
I've seen other cows with big udders that didn;t milk diddly and the udder looked pretty much the same after milking. Meaty udders like that will fool you. The proff is in the pudding, how does the calf grow or in the case of a dairy, how do the DHIA records look.