The rules don't specify a caveat to that beast-on-monster interaction.
Since it doesn't specify, the chance to knock down the target is present.
Does that make sense to you and your DM?
If yes, go with it.
If not, consider a related ability: the Shove action from the PHB(p.195) which specifies that the Strength check attempt can only be used on a creature up to one size larger than you to shove it away or knock it prone.
If medium, up to a large; if large, up to a huge, if huge, up to a gargantuan.
In this case, Large Dire Wolf versus Huge Hill giant, it's still a roll for a possible knockdown. (Dire wolves are sorta badass, eh?) A medium sized beast (boar) could not do that.
I've seen a ruling that if you go one size up your Shove attempt is at disadvantage, but as I can't find the source that may have been a table rule. (@DerekStucki advises that this idea was floated during play test, and was in the 1st printing PHB for the grappler feat, but has been errata'd out. It said, under the grappler feat "Creatures that are one size larger than you don’t automatically succeed on checks to escape your grapple." but the errata says "this refers to a non existent rule").