Lifting Strength could be calculated from size with only clearly defined assumptions. While it is intuitive that characters have to lift their own body weight to move around, lifting strength is the weight that they can lift in addition to that. Like any other characters, a lifting strength level has to be shown, or at least reasonably deduced, to be considered valid. Therefore, the character should demonstrate itself to be capable of performing a lifting strength feat to have the lifting strength tiered to the yield of the feat in question.
It is worth noting that there are cases where giant characters have to push around other giant characters or engage in physical brawls with them. In such cases, the masses of the combatants involved can be taken as lifting strength yields. In a similar fashion, a giant character capable of doing push-ups, sit-ups or squats; jumping reasonably high, or performing strength-demanding acrobatics can have their own mass taken as lifting strength.
There may be evidence that the reference figures these giant characters scale up from can lift far more than their own weight (e.g. a centipede can lift up to 15 times its own weight). However, it has currently been agreed that such considerations should not been taken into account (i.e. characters only get their yields as if they are normal humans) unless there are very strong official statements, and knowledgeable staff members give particular exemptions or allowances.