The blooms are too small. How do you make them bigger?
How to Make Hydrangea Blooms Larger
Certified GKH Gardening Expert
Have you pruned your hydrangea? If you don’t prune at all, it may be something you want to consider. If you prune, your hydrangea stands a better chance of growing larger flowers. A hydrangea that isn't pruned tends to have smaller ones. Tips for pruning hydrangeas can be found here:
Barring that, you can give your hydrangea its best chance for big blooms by ensuring it is getting the care it needs with regards to watering, fertilizing, light, etc. - this article provides you with a plan of care:
I'm not an expert but do the roots have enough room in the soil?