Using 'morsel' suggests a very tiny quantity, emphasizing the smallness of the food portion. She was dieting and only ate a morsel of cake.
Pro Tip 2/3
Not a Full Meal
'Morsel' doesn't refer to a full meal but rather to a bite-sized or minimal piece of it. He savored each morsel of the gourmet chocolate.
Pro Tip 3/3
Literary Device
Writers may use 'morsel' to indicate scarcity or to highlight a character's hunger or desire. The poor child picked up every morsel of food she could find.