Hilarious things you've found in RAW?

[Vaern]

Checked the article in the Epic Level Handbook: actually, it weighs "only" 250,000 pounds, and 40'x40'x70'
With 112,000 cubic feet and 250,000 pounds - it's denser than Osmium (over x1.5)

Your math is off by a few orders of magnitude. I think you've divided the weight by 112 cu ft instead of 112,000.
Osmium has a density of about 1,400 lb/cu ft, and the measurements given here would result in the colossus having a density of only 2.2 lb/cu ft.
112,000 cubic feet of something that's 1.5x as dense as Osmium would be roughly 235 million pounds.

Plus, the space a creature occupies on the battle grid doesn't necessarily reflect its volume. A human occupies a 5x5 square and might be 6 feet tall, but you wouldn't say that this character has a volume of 150 cubic feet. An actual human has a volume closer to 2-3 cubic feet. And, since a colossus is described as being often carved as an exact likeness of its creator, we can assume that the colossus has exact human shape and proportions to make some rough calculations.
If we multiply a human's proportions by 12 in each direction, we end up with a volume for a 72-foot tall colossus being roughly 2.5 x 12 x 12 x 12 = 4,320 cubic feet.
250,000 lb / 4,320 cu ft = 58 lb/cu ft.
Granite's actual density is about 170 lb/cu ft, so even after drastically decreasing the volume of the colossus it's still surprisingly lightweight at just over a third of granite's real-world density.
Water has a density of 62.4 lb/cu ft, so the colossus is less dense than water and would float.