Spheres are a good packing configuration, but when you are packing them in cyclinders (many canisters) or rounded corner rectangular boxes with baskets (many other canisters) both of those being taller than they are wide, small increases in diameter can have large effects on the number of units which will be held in the use container (the canister).
The Dupla spheres pack theoretically at 120 units per liter, or about 454 per gallon. The Coralife pack theoretically (depending on which figure you use from their site) at 225 or 300 units per gallon. That theoretic packing should be in a spherical container, but the odds are it is not, but you can bet it is at least in a cube box (Dupla uses triangle boxes for selling - don't ask me why). The farther away the use container is from a sphere or cube, the greater the effect of the size of the individual units on packing. For the two units we looking at the smaller gets at least 50% more, perhaps twice the number of units in the same size container, with only a 15% increase in diameter from the smaller to the larger unit. This is not just the volume increases being a cubed function, so small changes in size are large changes in volume, but also how many will fit in the diameter inside the cylinder that is the canister.
Does that make sense? If you have kids, you can play with two marble sizes in highball glasses to see the effect of diameter restriction on packing. I used that with my son who was in secondary school at the time I was playing with this myself. He had a hard time seeing why I did not just use the same larger Dupla Biokaskade spheres I use in my W/Ds (where the towers are very nearly cubes BTW - good packing).
