In 1873 French scientist, Edouard Roche proposed that the Moon simply formed alongside the Earth out of essentially the same nebular cloud of particles and gasses.
Scientists have had a basic understanding of how the solar system formed since at least 1755, when the philosopher Immanuel Kant proposed the nebular hypothesis.
The nebular model tells us that, back when Mars was forming, there should have been enough material in Mars's part of the disk for it to grow much larger.