I don't know any commercial products that have either an ATMega328 or ATmega2560. However a quick look puts the ATmega2560 nearly 4 times the cost of the 328.
For the OP; A new product designer has access to (then) Atmel and Microchip's tech rep as well as the tech rep for a dozen other microprocessor suppliers. So the just don't "pick" a processor the weigh functions with package size and cost.