Aldi is about 15% cheaper than Walmart according to Wolfe Research. Aldi has both fewer SKUs and a higher proportion of private label brands. Has smaller stores. And aggressive staff-cutting by having customers bag their own groceries, have return carts etc. One interesting optimization is having multiple barcodes on products to make scanning easier.
By comparison Walmart has 5355 locations, Safeway 2280, Target 1800 and Whole Foods 500. See