The SIZ having an automatic six figured in, I think, is to leave room for smaller races at the low end of the spectrum, not to boost player stats, as an automatic 6 would for, say, Intelligence.
For instance, if a human can get down to size 3, what would your typical hobbit / halfling / other shrimpy fantasy race be? I think the goal is to have the humanoid-ish races fall into the humanoid-ish range of values for SIZ, and for humans to be on the taller end of that spectrum.
I think this is also to place a correct metric on SIZ for other objects, so that you can generically measure the size of things in SIZ points and have them be comparable to the SIZ of a human. For instance, in the second edition RQ, when we still had opposed characteristic rolls, this rule put human SIZ values on the same scale as STR on the resistance table, so that you could do a simple STR vs. SIZ resistance roll to try to lift something of a given weight relative to the average human.