Well for one, Venom's strength range isn't a consistent 20 ton level, it's probably much more appropriate at 40 tons and increases with rage/anger. So there's no telling how strong the symbiote could become, but it's pretty clear Gargan has incredible fits of anger and hostility, and Spiderman has noted that the suit has made another, weaker host who was much more in his right mind as strong as the Thing.
And, didn't they fight 3 times?