How is eating food a physical change?

Actually, eating the food involves both physical and chemical changes. The chewing of food is a physical change, while the mixing of saliva with food triggers chemical reactions and is a chemical change.
A physical change may involve a change in the size, shape or state of a substance; however, its chemical composition stays the same. Chemical change, on the other hand, involves a change in the substance's chemical composition.
When we "eat" the food, we start by chewing it in our mouth. This action causes the food to break down into smaller portions, each of which is still food. In other words, although the size of the food changes, its chemical composition stays the same. This is the part of eating that represents a physical change.
Once the food mixes with the saliva in our mouth, chemical reactions start taking place in the food, leading to its breakdown and changes in its chemical composition. An evidence of the chemical changes is the taste that we feel when we eat something sweet or sour, etc.
Hope this helps.