We used to run a campaign of this at my club and the whole thing was dogged with how messy the rules are. It was a while ago, but I think that particular rule constitutes weapons like Heavy Bolters, Lascannons, Heavy Stubbers, etc, but we were fluid with our interpretation. So come up with a house rule based on common sense I suppose. If the gun's larger than an assault rifle and not carried by a space marine (in which case is it larger than a normal human...) treat it as heavy. Oh, and just general rules for the game that I remember: Squigs are bent as hell, so limit their use, and nobody is allowed to equip more than a quarter of their models with bionics unless its fluffy. Its easy to make an overpowered list, so its up to the players to reign themselves in (though I haven't played in about two years, so I don't know if that has changed).