Hmm... Carry capacity STR x2 ENC? Might be worth thinking about.
I think the problem of adding ENC values is another complication of the MRQ system. Really, 20 ENC n items won't require twice the STR to carry as 10 ENC n items (assuming these are packaged in a handy carrying case).
I'm not really sure how ENC
should be compared to STR. Another reason for not using it, except perhaps as a measure of "unwieldliness". (A weapon used by a giant might have an ENC of 15 for example, making it impossible to wield for normal humans, and very difficult even for very strong humans.)
Otherwise, it may be possible to rule carry capacity equals STR ENC, with ENC values calculated accordingly. The most important thing to consider is that it probably will not work out to add 10 ENC 1 items and get an ENC 10 items.
Back to SIZ, I think one concept is that if the items are NOT in a handy carrying case, a large creature has an advantage. A giant could easily put several boxes in his hand that a human would individually consider bulky. A human who somehow had the same STR (through magic) might have a very very difficult time carrying these same boxes all at once because he'd have to carefully balance the stack, even if the boxes were really mostly empty and very light.
But then it probably makes more sense to do what one D&D GM did, and just say "I may ask you hard questions [about
how you are carrying your items]."