Hello from JustAnswer.
When someone has always has small stools with gas, or at least many years of this, the most common cause would be because of muscular activity in the gut that is affecting the size of the stool the accumulations of gas. Everyone has ongoing muscular activity in the gut, which is what pushes material downstream. In some people the amount of muscular activity can be higher or lower than usual, which can affect stool formation or the accumulation of stool.
It is also possible that a single event that occurred when you were young may have altered the anatomy of the rectum and anus that formed a tight passage, and in turn forces the stool to be small to get past the tight passage. When it has been like this for a long time and is not associated with any pain or discomfort or difficulty in passing stool, we usually are not concerned.
When there is a recent change in the size of stool, that is actually more worrisome, as there may be a growth in the rectum that is also creating a tight passage. But the worrisome growths would not have been present for many years without progressing or causing other problems.
If this is making you nervous and you find that you are having to force a bowel movement, the usual first step would be to increase the amount of oral intake of fiber to 25-35 grams per day. The fiber can either be in foods, such as fresh fruits and vegetables, legumes, and whole grains, or via fiber supplements. The fiber is a bulking agent for the stool and can help to regulate the muscular activity of the gut. For the fiber to be effective, it is also important to drink plenty of hydrating fluids, which means that fluids that contain caffeine or alcohol do not count. The goal would be the intake of two quarts/liters per day. The fluids allow the fiber to work, as the fiber needs to absorb water to be effective.
If I can provide any further information, please let me know.