I'm very much of the "language is defined by use" philosophy. (Although it is important to note how exactly they are used. Words which are widely used but also widely regarded as wrong like irregardless, for instance, should be avoided.) Language evolves so much that any attempt to be a purist about language tends to collapse when facing the baroque realities of etymology. The purpose of language is to be able to communicate. As long as everyone agrees on what the language means, communication can go on easily. Purism is just needless conservatism.
But more to the point, you are factually wrong. I have access to the OED because I'm a college student, and thus can look up things like this. The word massive has been used to mean merely big (or things of that sort) for about as long as it's been used. One such example the OED gives (not the earliest, but the earliest that is clearly Modern English) is "The griping critic..wades through massive volumes in search of faults." written in 1756. Way back in 1420 is the more archaic "Among wer medled..whyte perlis massyf, large, & rounde" which is the oldest usage of the word and doesn't exactly sound like they were talking about density. (Although I don't fully grasp what is being said.) So booyah.