The PHB states:
The target of your grapple must be no more than one
size larger than you
Makes sense: being massive makes you hard to grapple. But does being massive also make it hard to resist your grapple?
Does being any number of creature-sizes larger confer any benefit to a grappler's grapple attempt? Or alternatively, does the target have any disadvantages contesting the grapple, or trying to break a grapple, from such a grappler?
I understand that a smaller target can make a Acrobatics (Dexterity) check, and by virtue of being more agile, break free from a vastly larger and stronger grappler. But what if the target is not agile, and uses an Athletics (Strength) check to contest the grapple attempt, or break free from it? Think a Hill Giant grappling a decently strong gnome (that's a difference of 3 sizes!).
What really got me wondering about this was the Enlarge spell. The Enlarge spell grants a bonus to Strength checks by virtue of increasing a creature by one creature-size. But does being naturally any number of creature-sizes larger than your target, confer any benefit to Strength checks (like grapples) against the target at all?
Best Answer
It does not grant extra benefits
The grappling rules do not say anything about size other than that you cannot grapple something two or more sizes larger than you, and that your speed is halved during a grapple unless the grappled creature is two or more sizes smaller than you.
By RAW, no advantage due to size is granted for any grapple checks.
The Enlarge/Reduce spell has multiple effects. One is to enlarge or reduce the size of the creature. Another is to grant advantage on Strength checks and saves. That is, the increase in size does not cause the advantage to Strength checks and saves. Both effects are caused by the magic of the spell.
In the spell description, notice how the advantage benefit is written. It is said as "also, you have this benefit" and not "because of that, you have this benefit" -- suggesting non-causally related items in a list.
If it was the size change that conferred the benefits of advantage, then a creature who was Medium size and enlarged to Large size, when grappling a creature of Large size, should not get advantage on their checks. But they do, because magic, literally.
Bigger monsters typically have higher Strength scores
In a way, being bigger does have advantages for the sake of grappling. But this is reflected in their high Strength scores and proficiency in Athletics check (if they have it). It is not reflected by any application of the advantage/disadvantage mechanic for reasons owing purely to size.