Why is my Hydrangea staying so small?

About a year ago I bought two blue hydrangeas. I've never grown them before. They both ended up covered in powdery mildew before I knew what it was and lost most of their leaves. I do know how to treat it now, and both plants are putting out lots of new leaves. I understand that these flower on "old growth" from the previous year and it looks like my little plants are going to have tons of blooms. But I don't see anything that looks like "new" growth on either. Just leaves and flower buds on old stems. Why don't my plants get any bigger/taller? How much should I expect them to enlarge each year? I live in South America, so we are just coming in to spring. Any information is greatly appreciated!!