Grapple Saving Throws

Does you roll the saving throw to resist being grappled, at a disadvantage if the creature attempting the grapple is 2 sizes or more larger? For example, a Large creature grappling a Medium creature. 

No since the rules don’t say so and it would significantly buff a lot of larger creatures with grapple abilities like the Purple Worm.

This would be absolutely brutal for a character that is neither Strength nor Dexterity based. Your poor Life Domain Cleric would have almost no chance of breaking loose if he gets grabbed. Don't use that rule.

While using Unarmed Strike to grapple is possible only if the target is no more than one size larger than you, size doesn't any impact on the saving throw for Ending a Grapple.