That is true because 5 Marla is not very good for house construction. Simply too small. 35x70 is ideal for investment in my view. And 40x80 is the second best for investment then 50x90. You can go for 60x90 if you get it cheap which is a good possibility. Some people need money and dispose of it at 50x90 price.
But generally larger plots are not fast moving items.