hi there. I had a subtotal colectomy on 9th Jan and am in the same boat. Sometimes it is like water and sometimes I even get constipated after eating the same foods.
I also have piles
I don't know about not eating large portions, I can eat LOADS but also sometimes get gas problems and really miss my fruit, veggies and salads.
I find things like mayonnaise help to thicken stool, eat it with chips, or tuna etc and a banana every day. Also stewed apple is easy to digest it produces and enzyme when cooked that can help apparently.
I did find a useful link but don't have it to hand, I shall come back with that. :)