Merry, in the chapter "Flotsam and Jetsam," is describing how Saruman's
forces set forth to war.
"I saw the enemy go: endless lines of marching Orcs; and troops of them
mounted on great wolves."
(This would be where PJ got the idea for his little extraneous skirmish where
Aragorn takes a dive. But note that it would take an awfully "great" wolf to
hold a man w/o his feet dragging. Jackson's wolves looked more like hyenas,
though, maybe he thinks that is what a Warg looks like.)
"And there were battalions of Men, too ... Most of them were ordinary men,
rather tall and dark-haired ... But there were some others that were
horrible: man-high, but with goblin-faces, sallow, leering, squint-eyed."
(If "goblin" means "orc" here, as I assume it does.)
In the earlier chapter on the "Uruk-Hai" it does appear that the Uruk-Hai are
larger than "ordinary" orcs. Are they the same as these "others that were
horrible"? Yet Merry does not verbally identify them with his captors.
And, at the end of the chapter "The Black Gate is Closed," Gollum describes
the "men of the South" as "almost as bad as Orcs, and much bigger."
I was tallying up evidence that Orcs were smaller as it might explain to some
extent how Men could slay such quantities of them; though I am not sure
Tolkien ever gives such exact (and daunting) odds for Helm's Deep as the "300
against 10,000" given by Jackson.