The size of an atom is primarily determined by the number of electron shells, not just the number of protons or its mass. Sulfur (S) has an atomic number of 16, meaning it has 16 protons and 16 electrons, while silicon (Si) has an atomic number of 14, with 14 protons and 14 electrons.
Both elements are in the same period of the periodic table, which means they have the same number of electron shells. However, as you move across a period from left to right, the number of protons in the nucleus increases. This increased positive charge in the nucleus exerts a stronger attractive force on the electrons, pulling them closer to the nucleus.
Even though sulfur has a greater mass due to having more protons and neutrons overall, the atomic radius of sulfur is smaller because its electrons are held more tightly by the stronger nuclear charge compared to silicon. Thus, the size of sulfur atoms is influenced more significantly by the effective nuclear charge than by the total mass or the total number of particles.