We've seen them at our local grocery and said the same thing. Not sure they aren't genetically modified so we buy Organic when possible since my toddler loves blueberries.
I work with a few traditional plant geneticists at my workplace (meaning that I am not an expert in the field).
These two links below back up my understanding about GMO fruiit/vegetable produce (not food crops like corn, soybeans, etc.)... there isn't much GMO produce on the market yet.
In other words, those large berries are most likely the result of discovering naturally occurring large berried plants and then propagating them through traditional genetics such as cross pollinating, cuttings, progeny tests, etc..
Hey I will have you know my Fukishima edition Nissan is doing far better then avg for the brand, besides Atomic Blueberries probably cost extra at Wholefoods.
There are many different varieties of blueberries, they mature at different times and some are more appropriate for certain climates. All are different sizes and have different flavors.