"Wow.. you wouldn't think 4 ounces would make that much difference in thickness"
--> It's a strange thing to describe the thickness and feeling of denim just by the weight. For example, I found, that the 19 oz Samurai denim didn't feel much heavier than the 17 and even the 15oz denim. The 21 oz is remarkably thicker and again, the 24/27 oz denim felt much heavier.
And my 14-14,5 oz LVCs feel much lighter than my 15,7oz Momos.
So, there's much more than just the weight, like G said, these 25 oz IHs seem to be very stiff and tightly woven, which may make them feel even heavier.