What causes a gall bladder to become enlarged and have lots of stones in it?

My sister went in to have her gall bladder removed. The surgeon was amazed at the size of it, it was the largest he's ever seen,and it had a ton of stones in it. My sister is obese, wieghs about 325 pounds. Would her weight cause this, or would it be something else that caused this?
Update: There is no history in my family for having bad gall bladders and stones.
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