Very interesting observation. Overhead watering is the only kind mine get right now so I can't comment. However, I am changing my irrigation system to allow for some hydrangea targeted drip watering next season and will be very interested to see if there is anything to this.

I'm sure you've taken this into account but since you didn't mention it, I have to ask if lighting conditions are equal?.....yg

YG, of course, I'm comparing apples to apples, though minimal variances still exist. To make comparison fair I also compare the SAME cultivars having different watering regimens, in order to avoid unfair comparison of plants with different habits, but in general ALL dreep-irrigated plants are wider at the bases and overall than overhead watered ones, being about the same time in a ground.
Basicaly, I have no problem to explain this phenomenum, but would like to hear from other people to confirm that this is a general pattern, not a local or incidental issue.