How much concrete to order?
If each statue is 1.25x as large as the original, its volume will be 1.25 cubed, or a little over 1.95 times the volume of the original. If you can find out the weight of the original and the specific gravity of the stone of which it is carved, you can calculate the volume of stone in the statue and use that to calculate how much concrete you would need for a statue having almost twice the volume. I'll defer to the concrete experts about particulars on that subject.