Whats the seat tube angle on the 53 cm frame. If you assess it carefully I think that you will find next to no difference in size between the 50 cm and the 53 cm except that the steeper seat tube angle on the 50 cm frame makes the top tube length look shorter on paper which is the EXACT reason why they do such a thing. By the time you fit a rear set seatpost to the smaller frame to position yourself back at KOPS there will be NO difference anymore.
When you get down to such small frame sizes it highly likely that you will be better off looking at 650c frames and I know there's a lot of prejudice against 650c wheels but that's the way it is. If you think to yourself, I don't want to ride a 650c bike then you know EXACTLY the reason why manufacturers fiddle with the geometry of 700c bikes to make it look right on paper.
It won't be a comfortable ride with such a steep seat tube angle of 75º but your more likely to buy it which is why manufacturers are more likely to make it.
Regards, Anthony