SIZ (since you're using BRP/RQ) also has a bearing on carrying capacity. A 25 kilo load to a 90 kilo carrier is less of a burden than it is to a 60kg person. Suggest half the Size score's variation from average times 10% as a modifier to the base of their Con/Str-derived capacity... So Siz 16 adds 15% (if Siz is 2d6+6).

Or:

Base it on CON and use the average of Str+Siz to modify it.