weight doesn't seem to factor in with amphetamine-based meds from the information I have. However it is usually the prudent thing to start low and work up. Many MDs don't want to wait and would rather have the patient walk in panicked and sweating from too much and then drop it until the side effects go away. My MD and therapist together put me on low-dose dexedrine to start and slowly brought me up to my present level. It makes more sense really.
Watching weight in children is highly recommended in the material sheets the doctors get. With Ritalin I believe it's nearly mandatory in younger children. There are a small group that have weight loss that's extreme (same with loss of growth in height) but has to be monitored.
Remember - the MD works for you so don't just accept what they say and take it! If you feel it's not right get another opinion. People sometimes are scared to ask or change things - but the doctor needs to get feedback or the treatment may go very wrong.