There is no size limitation to the normal grapple rules.
As long as you take the necessary bonus\penalty for sizes nothing prevent a smaller creature grabbing hold (and later even pinning) a larger one.
remember starting a grapple is not the Romen Greko wrestling pins. it's mostly just grabbing hold of the creature.
on that matter opposing str checks which are not maneuvers also do not have size limits.
Just last night in our game the party's pesky npc follower, a Pooka (tiny, str score 10) won an arm wrestling contest against a Brass Golem (Huge, str score 32).
she did roll a nat 20 but as a check i ruled it's not an auto success. the golem rolled too low so in the end she won.
Golem was ordered to test any who challenge it. the party tank already won, but the Pooka just couldn't stay still and challenged it as well. every1 was shocked when she won. the challenger needed to approach the Golem and state "behold my might", there will be no living with her after this...