Subject: Why equal number of molecules of two different gases occupy same volume?

An explanation I read from Chemistry - The Molecular Nature of Matter and 
Change Second Edition by Silberberg pg202-203 is that it is because that 
all the gases particles have the same average kinetic energy, therefore 
creates the same pressure. However, if one thinks carefully, there is 
something wrong with the statement. If the light and heavy particles have 
the same average kinetic energy, the heavy particles will travel much 
slower and hence create a lower pressure due to the infrequency of 
collision.
I understand that this phenomena is proven expermentally but what is the 
theorical explanation. Does it have to do with the intermolecular forces 
and gravity exerted by the mass?

Re: Why equal number of molecules of two different gases occupy same volume?