What is the primary cause of the differences in composition and size between the inner and outer planets of the Solar System?

The main reason the inner planets are small and rocky while the outer planets are huge and gaseous is the temperature gradient in the early Solar System. Near the Sun, high temperatures kept volatile molecules like water, methane, and ammonia in the gas phase, so only solid, metal‑rich and silicate materials could stick together, producing small, dense planets. Farther out the cooler temperatures allowed these volatile substances to freeze into ices, which helped collect large amounts of gas and ice into massive, gas‑giant planets. For example, at the distance of Jupiter, hydrogen and helium could condense and accumulate, whereas at Earth’s orbit only iron and rock could survive. Thus, the distance‑dependent temperature and material availability created the stark differences we see today.