After a while, hydrangeas have to be rejuvenated using a three-year
long process. On year one, prune down the 1/3 longest stems. On year 2,
prune down the next 1/3 longest stems. And in Year 3, prune the
remaining ones. If the bloom size remains consistently small, that could
be the issue.

Pruning the stems a lot might also produce smaller blooms from each of the branch ends of a stem. In total, doing that would cover a larger area with more blooms but the size of the blooms would be smaller than, say, if the bloom were growing on a single stem with no branches.

But I would also check the soil first and make
sure there are no insufficient levels of phosphorus, potassium, nitrogen
or minor minerals. A soil test can reveal this type of problem.

Over-fertilzing can also burn the roots, which, in turn, causes flower production issues.

Lastly, lack of sun can cause problems but you need dense shade for that and I suspect you would have mentioned that already. If grown in dappled sun or in full -but bright- shade, you should have still gotten normal sized bloomage.

How has water been for you this year? New England has been quite dry, and water stress can effect many aspects of a plant's development.

You might want to add your zone and location as Luis and I have. We are sort of at the opposite ends of Hydrangea growing, so our experience can be quite different, and we don't know where in the world you are. Here's how to add it so it posts by your name whenever you post:

Go to Your Houzz in the upper right of every page, click Edit Profile, and on the left side click advanced settings. Well down the advanced settings page is a blank labeled Climate Zone for Garden Forums along with a link to find your zone.

If you add info on your state or the nearest large city you will get even better information since zone only relates to average coldest winter temperatures.

Then return to the top of the page and click Done Editing.

Thank you both. My zone is USDA 8a. Near Salem, Oregon, which is a bit warmer than Portland, but also very mild and maritime. The type of mop head is King George. The blooms started out pink, but over the years have ended up a mix of pink and purple, as our soil is acid. Ive enjoyed that. Its been very pretty. But, as I say, they have been much smaller this year, and come to think of it, the color has been muted too.

No weather oddities this year, I think Ill try the fertilizer idea. thank you, hobbit mom.