Q.Why Are My Poinsettia Leaves So Small?
My Poinsettia is very healthy & started from a store bought Poinsettia with big leaves. Now it is healthy, with green leaves, but small leaves.

Certified GKH Gardening Expert
Was it started from a cutting or seeds? If it was started from seeds you can get a different variety than what the parent plant was.
If it was propagated from a cutting, then you need to review its best care practices. For example, lack of fertilization or not enough light could result in small leaves.
Here is more:
https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-plant-propagation.htm
https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-care-how-do-you-take-care-of-poinsettias.htm
https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/planting-poinsettia-seeds.htm
https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/how-to-make-poinsettia-turn-red-make-a-poinsettia-rebloom.htm