Thats a question someone asked me recently and yeah, this is especially true during bulan puasa where we fast all day and break with a heavy meal. So the question should actually be:
Why do we feel drowsy or sleepy after a LARGE meal!?
Lack of oxygen!
Its simple really. Eating a large meal makes your digestion system work overtime, this draws blood away from other areas, including your brain. Less blood flow reduces the amount of oxygen the brain gets. Hence you feel sleepy.