Thanks for your query.
I will try my best to provide you with best possible professional recommendations to your questions.
Ispaghula that is fybogel has nothing to do with the small volume of stools. Side effect of it are Flatulence
(excess wind), bloating
, abdominal discomfort. Small stools may be due to the dietary alteration. Do remember that when you consume fybogel it should be taken with sufficient quantity of water, never to be consumed dry.
Small stool presently does not indicate any grave problem, for your own satisfaction you can consult your physician, but need not to stop the drug for present symptom.
Yet again, I do hope that you have found something helpful and I will be glad to answer any further query.