Hello, Jen. Newly planted hydrangeas can produce small blooms until they get established. Since you planted it last year, I would just 2-3 years for it to settle down and grow a larger size root system. If by year 3 it persists, I would look at other causes like too much fertilizer. Also, minimize any pruning (it is ok to prune dead wood at any time though). Looks good otherwise!!!

Blooms coming from previous seasons' wood (lateral buds) are normally smaller, while terminal bud flowers (and flowers on new stems) are larger.