I too seem to recall some reasoning along the lines of 'certain weapons more useful against larger+ opponents'. This, IIRC, worked along several axis of development: usually pole arms (and 1e had a lot of pole arms) got the bonus, as one would expect your long spear to be a more effective weapon against an ancient dragon than the tiny, pin-prick like (to a dragon) weapons usually employed effectively against medium sized humanoid opponents by adventures; even in our own history, no one went wild boar hunting with a sword/dagger/mace. This seemed to help along the verisimilitude/realism axis, though YMMV, etc. Next, along with weapon vs AC rules, it did add some mechanical diversity to the weapons, so it wasn't always a 'long sword is best in all situations' optimization; though that long sword paradigm was probably at least partially intended due to the class restrictions on weapons in place at the time.
Of course I cannot post any links or quotes to validate the above reasoning, it has been too long.