Yes, my plan B was to just use an image and link it to my Twitter account, however there is more functionality to the Follow buttons provided by Twitter and translated into WP sourcecode on this page. Namely, those don't just open a new page to your Twitter account, it actually signs the clicker up as a follower automatically and opens a neat little pop-up rather than directing away from your blog.
All of that is just nice and not necessary, so I can just use a regular old image. I still think it would be very cool of WP to include the large format button. They have all the other variations of the official Twitter button (like whether or not you want your username or follower count displayed), so I'm not sure why the size choice was omitted.