The hamster cages aren't too big - the shelving unit is too small! I don't envy you - men can be very single minded sometimes. If we get in a stalemate situation I sometimes say something like "we'll have to agree to disagree on that one - you use small cages if you think that is right, I think a minimum cage size of xyz is right so we need to find something that will suit both of us".
How about - a long low shelving unit, for his cages, that will stand a detolf on top, that you can divide into two for your two hamsters? Sounds space-saving to me
And it'll look really nice on the wall (tell him that!)