This indicates the mass of a character. It affects his ability to do and absorb damage. Both large and small SIZ Adventurers have certain advantages. Large persons can absorb and deal more damage, but find it hard to defend themselves and hard to skulk in shadows. Small people have the opposite benefits and drawbacks. SIZ cannot be naturally altered.In the pages that follow that description, there are many examples of these "certain advantages." For example, a very small SIZ character gains a +5% bonus to parrying attacks, while a very large SIZ character suffers a -5% penalty. On the other hand, a high SIZ grants a bonus to hit points, while a low SIZ exacts a penalty. Other examples following this pattern abound.
I have to say I really like this approach to ability scores. Ever since I noticed that, in Space Opera, having a low Empathy score can be beneficial in certain circumstances, I've been thinking that it'd be really interesting if ability scores worked similarly in more RPGs. That is, instead of granting a bonus only for a high score and a penalty only for a low score, what if there were bonuses and penalties associated with each end of the range? Suppose -- and this is just an idle thought -- that, in D&D, having a high Strength granted bonuses to hit and damage in melee but also imposed penalties to Armor Class, on the assumption that a very strong character is much more massive and less flexible? Suppose -- and, again, this is just an idle thought -- that having a low Intelligence, in addition to limiting a character's ability to read and speak, also made him more resistant to a magic-user's spells?
I don't like the way that, over time, ability scores in D&D became ever more important, to the point that the AD&D Players Handbook stresses the necessity of having scores of 15 or more in "no fewer" than two abilities. At the same time, I think that abilities in LBB-only OD&D are too sketchy and barely have any reason to exist as discrete mechanics -- one might as well randomly roll for a "Earned XP Bonus" and be dispense with ability scores entirely. But an approach like the one that RuneQuest adopted, extended somewhat, seems potentially fruitful to me (though it is worth noting that, unless I am mistaken, this approach was dropped in most other iterations of BRP in the years since -- is it found in any contemporary version of the rules?). It'd simultaneously end the tyranny of high ability scores and put an end to the notion of "dump stats," since there'd be trade-offs regardless of the extreme to which your character's ability scores tended.
It's worth pondering anyway.