OK... let's look at some facts:
1) Below is a real modern day Asian elephant to human comparison. This is the starting point.
Look at the relative sizes in real life.
2)
Tolkien: "...his kin [the mumak's] that still live in latter days are but memories of his girth and majesty."
Ergo the mumak was larger than the pictured Asian elephant. The phrase "girth and majesty" coming from Tolkiens' pen, you can be certain he was NOT saying a Mumak was only perhaps 10% larger.
3) Olorgando's quoting the mass [volume] to surface area :
"That old saw that volume and mass increase to the third power (cubed), while area, here relevant for bone and muscle and ... cross sections only increase to the second power (squared). Make something twice as large results in eight times the volume and mass, but only four times the area - body surface (causing heat problems) and bone, muscle, ... cross-section" etc
is irrelevant as I doubt that Tolkien bothered about such "bio-physics" anymore that he bothered about them with respect to
Smaug's mass(volume) to surface area, which would have made Smaug's existence impossible.
4) related to (3)
It's fantasy ! Tolkien can make them as large as he wants. Now, were they PJ
Mumakil? I personally think not.
But again, I say, they were large enough to carry
towers and several
men on their backs.