Final answer:
A child would be expected to understand the differences in size and weight of objects during Piaget's concrete operational stage, between the ages of 7 to 11. Here, they develop logical thinking and master the concept of conservation.
Step-by-step explanation:
According to Piaget, a child would be expected to understand the differences in size and weights of a group of objects at the concrete operational stage. This stage, which occurs from about 7 to 11 years old, is where children develop the ability to think logically about concrete events. They begin to comprehend the concept of conservation, meaning they recognize that even if an object's appearance changes, its mass, volume, and number remain constant. For instance, they understand that if water is poured from a tall, thin glass into a short, fat glass, the amount of water remains the same. Children in this stage develop the skills necessary to perform basic mathematical operations and master the idea that transformations can occur without changing the total amount of something. This understanding of conservation is not present in earlier stages, such as the sensorimotor or preoperational stages, and develops further in the formal operational stage when children begin to comprehend abstract thinking and hypothetical scenarios.