If two bottles A and B contain I M and 1 m aqueous solution of sulphuric acid respectively,

1 M H₂SO₄ means 1 mole H₂SO₄ in 1000 cc of solution where as, 1 m H₂SO₄ means I mole H₂SO₄ in 1000 g of water
Total volume of 1 m solution will be greater than 1000 cc due to extra mole of H₂SO₄. Hence, number of moles per 1000cc will be less than 1 mole. Thus, 1 M is less  concentrated than 1 m.