Complex sentence: Pebble accretion becomes less efficient with increasing distance from the Sun due to the decreasing density of gas in the planetesimal disk, this slows the growth of more distant objects yielding a small Mars and a low mass asteroid belt.
Split into simpler sentences: Pebble accretion becomes less efficient with increasing distance from the Sun due to the decreasing density of gas in the planetesimal disk. This slows the growth of more distant objects yielding a small Mars and a low mass asteroid belt.