From the DMG:
Climb Onto a Bigger Creature
If one creature wants to jump onto another creature, it
can do so by grappling. A Small or Medium creature has
little chance of making a successful grapple against a
Huge or Gargantuan creature, however, unless magic
has granted the grappler supernatural might.
As an alternative, a suitably large opponent can be
treated as terrain for the purpose of jumping onto its
back or clinging to a limb.
The first paragraph of the action references the inability of normal-sized PCs from grappling Huge or Gargantuan creatures, but it doesn't clearly state whether the action can only be taken for those creatures, or if it can be taken on simply larger creatures. Throughout the rest of the section, it refers only to the difference between larger creatures and smaller creatures.
This begs the question, can a Medium-sized PC climb onto a Large opponent (think Legolas climbing onto the cave troll in Moria)?