If we can rule out improper pruning after flower buds set, I would suggest a problem preparing the bigger plant for winter. In that scenario, winter protection was enough for the small one but not enough for the large one. For more info on overwintering hydrangeas, check Winter Protection For Hydrangeas and Hayseedmans Overwintering Hydrangea Macrophylla Winter Protection Post.

Luis

Thanks Luis, these both seem like good & plausible reasons. But, I think we can rule out pruning after bud set; the only pruining I did was very early, as soon as I could tell which was dead wood from last year & which was viable. (One set was brown and sticklike and dead-looking, and one was green and pliable and young and growing...then again, maybe the brown sticks had potential. But I pruned the brown sticks off the blomming one, sigh.)
In terms of winter protection, if anything, the little bloomin' guy hard a slightly harder time, as he is by the corner of a fenceline & has to deal w/any west wind that may wrap around (tho it is just a 4' picket,) and the big ol'foliage guy is a teeny bit more protected, being about 8' further in.
Other than their relative location in front of the fence, both have gotten the same treatment.
Could I have pruned out all of the bearing stems (when they were still 2" tall) on 1 plant and none on the other?

Thank you for the references on winter protection. Just when I thot I knew something about gardening, I plant something new and start at the beginning!

Donna
in stormy NY