What causes occasional pencil thin stools, often the first stool in a bm is normal diameter, but everything else is increasingly thinner? concerning?

Might: Most likely for a person who is 19 year old and has no other sign or symptoms i would not be worried. Irritable bowel syndrome which is very common can be blamed. This also has been described in colon cancer but cancer is a disease which affects people over 50.

If you were: Older i would say there would be a concern for colon cancer. At your age, without a strong family hx of familial polyposis, this is not a concern. Why this is occurring could be worked up but the chances of an issue of cancer is very low probability.