Tolkein's orcs had big heads? That's a feature I missed too. What's your reference? (I'm not arguing, I just want to look it up!)
OK, first of all, it's a feature that's remarked upon in certain individual orcs:
The Great Goblin ('Over Hill and Under Hill',
The Hobbit):
"There in the shadows on a large flat stone sat a tremendous goblin with a huge head, ..."
Ugluk or one of his comrades ('The Riders of Rohan',
LotR):
"Upon a stake in the middle was set a great goblin head; upon its shattered helm the white badge could still be seen."
Grishnakh ('The Uruk-hai',
LotR):
"Dimly they were aware of Grishnak's great head and hideous face between them; his foul breath was on their cheeks."
Azog (Appendix A of
The Lord of the Rings):
"Thereupon Azog came forth, and he was a great Orc with a huge iron-clad head, and yet agile and strong."
There's also the orc-chieftain in Moria whose face is "broad".
Now, one might argue that these are distinguishing features of those individuals. But it is virtually every individualised orc in the first few books of LotR (and by the time we get to Shagrat and Gorbag, we've probably got a grasp of the orcish physique). But then there's this early draft of Sam and Frodo's adventures in Mordor, published in
The Treason of Isengard:
"The black iron cap was too large for Sam (for orcs have large heads for their size), but he slipped on the mail. It hung a little loose and long."
By the time of Tolkien's published version of the episode, of course, Frodo can find one helmet that fits "well enough", but by then the presence of smaller orcs in Mordor has been established (who knows - perhaps to facilitate both the finding of the disguise and its effectiveness when Sam and Frodo join the Durthang line?).
In any case, the
Treason of Isengard reference and all those individual big heads give a fairly clear impression.