Edward M. Condon, MD, FACE, ECNU
First, knowing the TYPE of diabetes is important. Type 1 diabetes is a lack of insulin and subsequent elevation in blood sugars. Type 2 diabetes is insulin resistance (can't use it) and subsequent elevation in blood sugars. Insulin is anabolic. This means it promotes the machinery of our cells to grow and divide. Hence, insulin deficiency (type 1 diabetics) are often quite thin in contrast to type 2 diabetics (make insulin) who are generally obese (or at least overweight).
Extremely elevated blood sugars, regardless of type, can put your body into a sense of starvation. Regardless of underlying type of diabetes, body perceives "low sugar" as "low energy" because it can't get into the cells for use. This prompts breakdown of muscle tissue and to a lesser extent fat tissue, causing weight loss.
For most people, good control is defined by a hemoglobin A1c < ~7% or avg sugar around 150 or less.
If you're on adequate therapy with good results, then I would take a quick look into thyroid and bowel habits. Make sure all else is well outside of the diabetes. There is no rule in medicine that states you can only have one disease process at a time.
A healthy body mass index (BMI) is 19-24.9 (unless Asian).
Doing a food/calorie log and taking it to your doctor can help guide you and provides tremendous insight into your day to day intake.