The other thing that will affect the size of your stools, will be what you eat and drink. If you are eating a lot of fibre and consuming a lot of fluids, which one often does on a diet, then the resulting faeces will be quite soft so that the sphincter doesn't need to expand so much because the resultant stool so soft it just slips through very easily.
If you think about the opposite end of the spectrum and you are constipated and passing hard large diameter stools, a lot of effort is required for the sphincter to stretch enough to let it through and it can be very painful.
Far from being a sign of illness, your 'problem' is probably merely the sign of a healthy diet.