In the case of the alkali metals

Gp 1 metals form cations M⁺ by loss of electron from outermost shell. Electronic configuration of Gp 1 metals is ns¹. When the outer electron is removed to give a positive ion, the size decreases because the outermost shell is completely removed. After removal of an electron, the positive charge of the nucleus is greater on the remaining electrons so that each of the remaining electrons are attracted more strongly towards the nucleus. This further reduces the size.