While GURPS has a lot of interesting rules for combat between opponents of very different size, it still has a few obvious scenarios which it doesn’t cover.
Squeezing
A bear hug typically needs a two-handed (or legged) grapple, but it seems viable with one hand if your opponent is small enough. Squeezing them between arm and body should be possible if your SM exceeds theirs by 4, while doing it with just your hand is only possible if your SM is 7 higher than theirs. The former requires first grappling an opponent, then getting them into position (treat as Shift Grip, GURPS Martial Arts p. 117), and finally squeezing. The latter simply requires a grapple followed by squeezing as your next attack. In both cases, use half your actual ST in the contest to squeeze.
Chewing and Swallowing
If your SM is 9 or more greater than your opponent’s, you can fit them into your mouth! Start with a bite, which establishes a free grapple (see Teeth, GURPS Martial Arts p. 115), followed by a ‘pin’. Once ‘pinned’ inside your mouth, they must hold their breath (GURPS Basic Set pp. 351-352), assuming they can’t breathe underwater. On subsequent turns, you can chew or swallow them. Chewing inflicts normal biting damage, in the same way as worrying.
Someone trapped in a mouth who manages to break free escapes from the mouth, but someone who has been swallowed is still trapped. They count as being grappled and can only escape by cutting their way out of their enemy from the inside! Treat this as an attack to the neck. The DX penalty for being grappled and the bonus for target size cancel out, so just roll against unmodified skill. Cutting, corrosion, or burning damage creates a hole large enough to crawl out of once your opponent suffers total injury greater than your hit points.
Born Biter adds to your SM for the purposes of fitting victims into your mouth and swallowing them.