A picture might help. A small, recently planted Annabelle might just need a few more years to produce larger blooms. It is called getting established.

It is hard to give an opinion of its size versus "it's peers" without a picture. Sparse and lanky might suggest it needs more sun, the best of which is morning sun and afternoon shade, or dappled sun.

You can feed it a 1\2 to 1 cup of organic compost, composted manure or cottonseed meal. Or a chemical fertilizer with a NPK ratio of 10-10-10. Just once in Spring but before July. I usually do it when they leaf out or a few weeks after my age date of last frost.

It's only a year old. You are comparing it to older, larger plants. IME, usually you shouldn't expect a whole lot from shrubs for about 3 years even when you are meeting all their needs. I view gardening is one of my life lessons in patience. Based on my Annabelle, there will come a day when you wonder why you ever worried about this shrub - mine is a monster if I don't stay on top of removing suckers, but it is about 15 years old. It's looked quite vigorous since it was just a few years old.

Mine is in its third summer and it is finally putting on some size! Years one and two it was puny. Very exciting to see how much it is growing this year.

I had the same problem last year with new annabelle. this year they tripled in size !!! and doing really well. forget about annabelle in the first year

prune hard next spring and you will be amazed about your plant

3 years seems to the the magic number on most Annabelles. This goes for just about any STARTER size. I started with bare root quarts, 2 plants, about 6 years ago. I now have 5. The two originals are 5x5' and the ones I divided off of them are about 3x3'.

Starting with 3-5 gallon plants gives you a much better start. Reduces the 'takeoff' time to 1-2 years.