? on muscles of the rectum

I have used suppositiories only a few times. The instructions on the container reads that the suppository should be inserted well up into the rectum. I have small fingers and have problems getting my finger inserted even up to the first knuckle of my middle finger. Is there some type of muscle contraction going on here that is keeping me from having a bowl movement? I think this constricting is normal at oher times. Maybe this is just how all of us are made!