Amazon.com Inc. is testing its cashierless checkout technology for bigger stores, according to people familiar with the matter. If successful, the strategy would further challenge brick-and-mortar retailers racing to make their businesses more convenient.

The online retail giant is experimenting with the technology in Seattle in a larger space formatted like a big store, the people said. The systems track what shoppers pick from shelves and charges them automatically when they leave a store. Although the technology functions...