Are large, flat colon polyps cancerous more often that other polyps?

Yes: Truly either pedunculated ( i.E with stalk) or sessile (flat) can harbor a malignant or pre-malignant lesion. The flat ones are a little more worrisome, and size of lesion also plays a role. Biopsy is the key to diagnosis - if the lesion is large and flat, there is always the possibility that malignancy may exist elsewhere within the lesion and surgery or close follow up may be needed.

Yes.: Larger polyps are more likely than small polyps to contain cancer or cells that are very close to becoming cancer. Flat polyps are also more worrisome, but it is not clear if this is due to the flatness or because a flat polyp is harder to identify on colonoscopy, especially if the colon prep has not cleaned the colon lining well.