Let's go to the chalkboard. (2) 1/8" shock cords have a combined cross-section (pi x radius squared) of .0245 square inches. (1) 1/4" shock cord has a cross-section of .049 square inches. To approximate the same cross sectional area of elastic between the two diameters, you would need (4) 1/8" shock cords.
I hope this helps once you wipe all of the geek off of it.