Grazzt said:
Technically though, being big (Large, Huge, etc.) doesnt necessarily mean you get a Str bonus. Look at the Otyugh for example: Large with a Str 11 (no Str bonus).
I base that roughly off the chart on Size Increases in the MM, in which moving from size medium to large grants (for example) a +8 strength bonus. This doesn't always apply, but the tendency is for big creatures to be substantially stronger than comparable medium-sized creatures -- compare the house cat (tiny, str 3) to the leopard (medium, str 16) to the tiger (large, str 23) to the dire tiger (huge, str 27). You can make similar comparison with different birds, with dogs, with monkeys, with monstrous humanoids, and so forth. It's not a hard-and-fast rule, but it's a strong tendency, that smaller creatures are weaker than similarly-shaped larger creatures.
On reflection, it's actually a triple-whammy: larger creatures also have more hit-dice leading to a higher BAB on the grapple check.
Your idea of giving an escape-artist size bonus might work -- I'm not really sure. The dex bonus that you get for being small doesn't come nearly close enough to providing a means of escape from a gbig creature, however -- in my experience, the double-whammy of strength and size increases make it nearly impossible for a grappled victim to escape a grappler even one size larger.
I do think it should be difficult, but I don't want it to be impossible, for a smaller creature to escape the grapple of a larger creature.
Daniel