The plants in the garden center in the spring are typically quite mature. As the season progresses and the older plants are sold out, growers then begin shipping new, younger crops, thus you see less mature plants. All are produced from cuttings, not divided a with a perennial i.e. hosta or daylily. I am guessing the last plants you described with 3/4" stems are not Hydrangea macrophylla but more likely Hydrangea paniculata which have larger stems as you described.

Tim, old thread but I'd figure I'd chime back in.

It did not look like it was a cutting, rooted and then established in pot for sale in comp. The pots I was trying to describe that looked -rushed to market- seemed to be more mature plants that literally have been divided a shark shovel or blade and just shoehorned into a pot