GRAPPLING (and Clinging):
There's no expertise on grapple checks:
1) For the purpose of grappling you can't have the benefit of expertise! So even if you have expertise in athletics, or acrobatics when defending. You'll only apply your proficiency bonus (x1), and not your full expertise bonus (x2).
Larger creatures than you:
2) You have disadvantage when grappling a target that's one size category larger than you. You still can't grapple a creature two categories larger and you still don't have disadvantage when defending against a larger creature.
The art of clinging to big guys:
3) You may cling to a creature that's two size categories larger than you instead of grappling it. You then move when it moves. The creature isn't impaired by your clinging. You don't get disadvantage on a clinging attacks because of size.