I'm just a bio major, not a doc. I'm going to take an intelligent guess here and say it has to do with diffusion. In diabetes, you're going to have an increased concentration of glucose particles in your blood since you have trouble getting them into your cells. So, now you have a large number sugar solutes in your blood, and when that blood is being filtered by the kidney, the concentration gradient would be such that more water would leave the blood stream and enter the portion of plasma being collected for urine than normal. So now, you have increased urine...^^. This is just a stab in the dark as I am way too lazy to google.