Just to update - prompted by a comment that sometimes vendors simply put a smaller plant in a larger pot with lots of compost, I called the company (begins with T-M) and asked how old the two different size pots were. The person I spoke to was very helpful about ideal planting needs and agreed the smaller_size would be better as it would be less "set in its ways" so to speak and so would settle in to a new location more quickly (particularly given the limitations of my tiny garden). Hopefully in a year or two I'll have a happy hydrangea, however big it is!