My buds starting opening on Jan. 3, and today they are blooming well, some of my little stems have two buds on them. However only one of the buds are blooming. I think that my flowers are small because they are in a pot. But otherwise I think that they are doing fine.
My zinnias are blooming
They certainly look happy and healthy. I may just be that cooler weather is slowing down the growth a bit.
Probably not an issue for you, but as long as they don't get frost, they should keep growing like crazy. Deadhead old blooms to encourage as many new flowers as possible. There are dwarf types of Zinnia, your plant may be one of those, perfectly normal sized. Looks great!