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Many creatures, such as the Giant Frog, have attacks that grapple on a hit:
Bite. Melee Weapon Attack: [...] Hit: (1d6 + 1) piercing damage. The target is grappled (escape DC 11).
Does this grapple work against a creature Huge or larger? Usual Grapples don't work if the target is more than one size larger than the grappler.
The target of your grapple must be no more than one size larger than you and must be within your reach.
When you want to grab a creature and hold on to them, the "grappling" attack option is how you accomplish that. The rules for doing this include a size limit on the target. If you succeed, you apply the grappled condition to the target.
But the size limit is a function of the grappling attack, not a rule that applies generally to the grappled condition. A condition is what it is, as defined in the rules appendix A (PHB p.290).
Spells, feats, magic items, or monster special actions may provide other ways to inflict the grappled condition without using the "grapple attack" rules, and therefore have no inherent size limits; the spell, feat, item, or action will specify what limits, if any, apply to it.
So yes, in theory, a giant frog could grapple an ancient dragon and stop it in its tracks. As a DM, you should feel free to overrule this if you feel it's absurd, but it's technically what the rules say.
The rules don't specify if the grapple is independent of size, but as the grappling rules for monsters say:
Many monsters have special attacks that allow them to quickly grapple prey. When a monster hits with such an attack, it doesn't need to make an additional ability check to determine whether the grapple succeeds, unless the attack says otherwise.
A creature grappled by the monster can use its action to try to escape. To do so, it must succeed on a Strength (Athletics) or Dexterity (Acrobatics) check against the escape DC in the monster's stat block. If no escape DC is given, assume the DC is 10 + the monster's Strength (Athletics) modifier.
Since there's nothing specific mentioned on the situation, I assume general grappling rules apply. So treat it as a normal grapple but without the need of a additional ability test.
Note that some monsters specify the size the grapple works on, usually when it's different from the general rule.
The grapple ability of many monsters do not necessarily work the same way as a PC grappling. Although the resulting grappled condition is identical, the process by which the creature gets grappled just follows the description as given. There is no general rule that says otherwise.
Note that the Giant Frog's "swallow" ability specifically targets a creature it is grappling but only if that creature is Small or smaller. So if size matters (so to speak), then it will say so.
