As we move from left to right within a period, the atomic number increases by one, which means the number of protons and the positive charge on the nucleus increase.
However, the additional electrons are added to the same outermost shell, so no new shells are added.
Due to the increased nuclear charge, the electrons are pulled towards the nucleus with greater force.
This reduces the distance between the nucleus and the outermost shell, resulting in a decrease in the atomic radius and the overall size of the atom.