Well, since someone mentions "old models" my spidey senses tingle, let me tell you my story about my old land speeders.. first of all they are the 1st edition ones, the infamous flying chairlifts, and they are about 60% the size of the current ones. I was in a tourney, and there was a large piece of terrain with a pipe all the way through it, and I decided that since my models fit, including bases, they would turbo boost into the pipe. My opponent, who was also marines, was cheesed off since his didnt fit. Whats the solution? I told him he could go into them if he wanted to.
Now, with true LOS, a smaller hive tyrant COULD get cover saves from termagaunts theoretically, but its just best to not take them, so all arguments are threfore null and void.. its as if your tyrant is actually taller. Or, you could do what i did for my Dreadnoughts (also old school): mount them on a heroic looking base, which is a bit taller up (on top of rubble, bodies, skulls, what have you) rather than on just the plain base, then it would actually be closer to the real height. Again problem solved. About the only cheesy thing for the smaller hive tyrant i could see is if you try and claim cover saves from infantry models, and do not have a gun. See, because TLOS comes to bite you in the ass when it comes to your model seeing other things, if it is lower to the ground, its tougher to see things: My Squat IG army has this problem for every single model, including tanks. And in the 7 years Ive been playing them, guess how many times the smaller height has ever come into play? 3 whole times.
So, in the end, model what you want, just make sure you're not the TFG by trying to claim cover saves because YOUR model is shorter. Let the other guy be TFG