You open 3 trades and double each time. 1 lot, 2 lot, 4 lot. That is a 7 lot position, if you stop increasing trades from this point, think of it as a single position of 7 lot with the price of the average price as the entry. You can limit risks with martingale as well by considering stoploss or % limit loss on the final position. And suddenly martingale and single position have the same risks.