volume of diatomic gas is less than the monatomic gas , although diatomic gas has 2 moles ( 2 times atomes) than monoatomic gas ??

volume of all gases under same pressure and temperature have same no. of moles
Hence the volume of each atom=average volume of the molecule of diatomic gas.
volume of each atom of the diatomic gas is less than the vilume of the atoms of the monoatomic gas.

the answer is derived by avogadros law where one mole has n(a) molecules and each mole occupies the same volume

volume of all gases under same pressure and temperature have same no. of moles
Hence the volume of each atom=average volume of the molecule of diatomic gas.