Skeletons have SM -1 because they're harder to hit, since the structure isn't all there. The same logic can be used for other creatures, e.g. the Elegant Spears of the Jungles of Zaridun, who have adapted to the heavy monsoons by becoming voluminous lattice-works of chitin to let the water flow through without resistance.
SM can never give more than a -4 or +4 in melee combat, and it will never cause the Size/Range/Speed bonus to be larger/less than
For example, fighting a Giant with SM +6 with a sword would only give you a +4 to hit, because after something gets large enough it isn't easier to hit it if it's larger. Is it easier to punch a barn than the Shuttle Assembly Building?
Similarly, if you shot an SM+10 creature from 200 yards away (-12), you'd be at an effective -2. If you shot the same creature from only 15 yards away (-5), you'd be at an effective +4 - the other +1 is lost, because after a certain point it isn't easier to shoot a larger thing. If you were shooting it from two inches away, you'd still only have a +4 - though I wouldn't make you roll unless there were serious other factors in play about whether you could hit it (which the rest of its SM could be used to help cancel out - it is easier to hit the Shuttle Assembly Building than a house if you're trying to do it in a gale).