Picking up a ring and guiding it onto the post offers repeated hand-eye coordination practice. Exploring which ring fits gives a young toddler a concrete experience of different sizes.
Start by taking the rings off together. Offer one ring to put back, then try another. Say big and little while exploring instead of insisting on the correct order.
Let the play grow with them
Keep the goal simple for a younger toddler. Later, compare two rings and let your child experiment with the order and the rocking base.
A no-shopping version
Nest two or three unbreakable bowls of different sizes.