I WONDER - Who Discovered The Brontosaurus?

The Brontosaurus, meaning 'Thunder lizard', was first described by Othneil C Marsh in 1877. Initially, Marsh named a short-legged dinosaur Apatosaurus. Later, he found a larger fossil and named it Brontosaurus. Scientists later discovered both fossils were of the same dinosaur, leading to the dual names.

What is the significance of the size in naming the Brontosaurus?

Why did scientists realize that the two fossils were actually the same dinosaur?