Some kinds like gherkins are smaller than the cucumbers that show up in the store, but people pickle the large cukes as well (more common as the dill or sour varieties). Also in other countries it's common to pickle all kinds of vegetables and even some fruit
I was much younger when I figured it out, but my mom also did a lot of gardening and canning when I was younger, so we had all kinds of pickled vegetables on the shelves.
There was still an aha moment when I realized that pickles were just pickled cucumbers, just like the pickled greenbeans, and pickled beets, etc.
I guess cucumbers were first on the scene and got to use the shortened name ;-)