Abstract In order to explain why the mass density of dwarf spheroidal galaxies is so low, the following hypothesis is proposed: dwarf spheroidals were born as dense systems but afterward they have expanded due to the supernova-driven gas ejection. Simple model calculations suggest that protogalaxies of 106−9 M◉ eject approximately half of their mass in a time scale shorter than the crossing time Tcr after they have settled down to virial equilibrium, provided that the time scale of star formation is ~4Tcr. It turns out from the virial theorem that such an ejection of gas results in a drastic expansion of the remaining stellar system.