Dave you're always telling me and Sammi to eat well now it's my turn - go get yourself something nice and nutritous. Will Mrs Dave not cook you up a stir fry or pasta or something?
You take in a small amount of food, your stomach produces more acid to digest it. Then keeps producing acid for a while even after it's digested, giving you that empty feeling. At the same time your body breaks down the starch in the crisps, then produces insulin to turn the glucose into glycogen to be stored in the body. The insulin keeps on working for a while, using up all free glucose in your body leaving you feeling a little tired. Same effect as with sugary foods only slightly decelerated. Ergo you feel hungry