I love that you did this test. Pretty much all the garlic I've ever grown started out as the cheapest, none organic store bought garlic. I'd just plant whatever sprouted when I didn't use them quickly enough and gradually I've been getting enough to start again from my own, home grown, supplies.
Bear in mind I'm in Australia, but my experience has been that imported garlic is treated and will rot rather than sprout if you leave it too long, whereas locally grown always sprouts. How well my garlic grows is generally down to the weather conditions. Last season we had a drier than usual winter and where other crops struggled, the garlic grew huge and even sent up staves for the first time ever. This year the garlic has been much smaller and didn't stave, as per usual.
I used to plant every single clove that sprouted, but now I have an abundance I try to use only the large cloves. I'm looking forward to seeing what your conclusion is on the difference in clove sizes.