What compositional difference between outer giant planets and Earth is key to formation models?

Relatively lack of heavy metals in outer giant planets compared to core mass estimates

The study of solar system formation relies heavily on contrasting the chemical makeup of the inner, rocky planets (Mercury, Venus, Earth, Mars) with the outer, massive worlds (Jupiter, Saturn, Uranus, Neptune). A crucial piece of evidence involves the composition regarding heavier elements, often referred to as metals in astronomical contexts. Observations suggest that the outer giant planets, despite their massive overall size, contain a lower proportion of heavy elements relative to the mass estimated for their solid cores compared to the composition of Earth. This discrepancy points toward the conditions present in the early solar nebula: the regions far from the Sun were significantly richer in volatile compounds (like hydrogen, helium, and lighter ices) which dominated the accretion process there, whereas the hotter, inner regions allowed only heavier, refractory materials to condense and form the terrestrial planets.