The reason is that Oxygen atoms contain 16 times more mass than Hydrogen atoms.
Hydrogen Has only one proton and no neutrons and so has an atomic number of 1 and a mass of 1
Oxygen Has 8 protons and also 8 neutrons and so has an atomic number of 8 and a mass of 16.
It takes 16 Hydrogen atoms to equal the mass of 1 oxygen atom.
