the(prefixed to a comparative means "thereby" or 'by so much', e.g. What student is the better for mastering these futile distinctions? This combination can enter into the further construction seen in The more the merrier (i.e. 'by how much more, by that much merrier'). It cannot enter into a construction with than: the tendency to insert it before more and less (putting any the more, none the less for anymore, no less) should be resisted, e.g. in The intellectual release had been no less (not one the less) marked than the physical. - The Oxford Guide to English Usage Alexander Demidov)
