It wasn't one single person who proved that objects fall toward the Earth at a constant acceleration regardless of their size. It was a series of discoveries and experiments by various scientists that led to this understanding.

Here are some key figures and their contributions:

* Galileo Galilei: Often credited with being the first to demonstrate this concept through his experiments with dropping objects from the Leaning Tower of Pisa. While the story of the experiment might be apocryphal, Galileo's meticulous observations and experiments with inclined planes showed that objects of different masses fell at the same rate in a vacuum. This was a significant step towards understanding constant acceleration due to gravity.

* Isaac Newton: Newton's Law of Universal Gravitation, which states that every particle of matter in the universe attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers, laid the theoretical foundation for understanding why objects fall towards the Earth with a constant acceleration.

* Later experiments: More refined experiments, like the famous feather and hammer drop on the Moon during the Apollo 15 mission, further demonstrated this principle.

It's important to note that the concept of constant acceleration due to gravity only applies in a vacuum. In the presence of air resistance, heavier objects will fall faster than lighter objects. However, the acceleration due to gravity (approximately 9.8 m/s²) remains constant for all objects regardless of their mass.