I am not sure what paf issb is or what relevance the size of an anus has to it.
The anus naturally tends to loosen as a person ages but this doesn't stop the sphincter muscle from doing its job of sealing in the contents of the bowel.
The default position of the muscle is closed and it is a very powerful muscle that will tighten even further when coughing, sneezing or doing other physical activities.
Those who have had extensive amounts of anal sex sometimes find the muscle has become so slack or lazy it can no longer seal the anus and sanitary protection must be worn against leakage, though I understand there is a straightforward surgical procedure to tighten things up or even replace the muscle altogether.
In that case a dormant leg muscle is coiled and transplanted and a pacemaker device used to trigger it for bowel emptying to take place.
If your anus isn't leaking you don't have a problem but if it is you need the services of a doctor.