The reason jeans had two big square back pockets was so when the time came to repair the knees the material required was at hand. So me mam always said.
Nice story, but if it was true, they wouldn't of rivetted them on, making the pockets almost impossible to remove.
Useless fact of the day - The original patent Levi's filed for the first denim jean in 1873 only had one back pocket, the second back pocket was added due to customer demand in 1901.