Weight Lifting is as much about grip and leverage as it is about the weight of an object being lifted. A person might be able to lift the equivalent of a small car's weight on a set of barbells but it is quite another thing to lift a car. On the other hand, how does a 10 STR normal who is 220 lbs. over weight even move will all that extra encumbrance they are carry? There are a lot of people who go about their daily lives every day carrying this extra encumbrance. I think there is a lot more about lifting and encumbrance than a bit of number crunching can solve. I think everyone should apply the common sense rule when it is required. As of yet, I have never seen a lone human being pick up a small car. I don't think it is possible. Humans are just too small to lift such a large bulky object. A quick and dirty rule may be to use a character's Casual STR unless they have a few levels of growth when trying to lift heavy objects larger than themselves.