Gallstones form in the base of the gallbladder from secretions of what is commonly called 'white bile'. They form because the gallbladder isn't working properly.
If the gallbladder is full of 'sludge' it can be seen on the ultra-sound as a fog on the image. In time sludge can cystallise (hardened). When they get larger they become cholesterol gallstones.
Gallbladders are usually a set size and it is unusual see a large or small one but they do exist. When the gallbladder gets choked with 'sludge' it is usual for the wall to be become thicker. This measurement is used to determine gallbladder problem.
The bile in the gallbladder can form grain sized globs of thick bile which are continually released into the intestines where they accumulate until they are absorbed. The Epsom Salts releases them with fresh ones from the gallbladder and you see a lot of them. Sometimes they are compressed by the colon to form larger balls consisting of bile grains.