You should look at your negatives. The negatives will show you the amount of space for each picture, so you can figure out if you are somehow taking larger pictures. It will also show you if you are misloading the film, and show no pictures for 1–7 or so. If you have no pictures on 28–36, it may be a camera option on the film size, but I haven’t heard of that before. I guess it is possible, though.