The background, size, and weight seem fine for me. You just need to add a comma before the "but even relatively poor" in the second sentence of the background.
The enhanced version looks fine, too.
Updating
Wooden Juggernaut Working Draft.
While doing so I fixed an error in the Grapple which should be +21 rather than +19. (BAB +3 plus Str +10 plus Size +8)
In terms of the red Construction text, I'm inclined to bump the CL to 9th and Craft DCs to 17. Do you think maybe an extra 1000gp per HD? I think I'd only add to the materials cost with an increase in size. I'm not sure about the extra cost per size category. Perhaps +10,000 gp for Gargantuan and +40,000 gp for Colossal with a correspondingly increased materials cost? Also, we should make the "if this increases the size" start a new sentence.
I'd be more inclined to make it CL 9th and DC 15. Making it DC 17 seems too high considering that's the DC of a Stone Golem, which has a significantly higher CR.
Speaking of Stone Golems, a Greater Stone Golem costs 105,000 gp to manufacture compared to the 50,000 gp of the base model. That's 55,000 gp more for one size category larger and 28 extra HD and a Challenge Rating 5 levels higher.
A Greater Stone Golem is that much better for an additional 55,000 gp then presumably a "max advanced" Wooden Juggernaut ought to be cheaper. Your proposed formula would make a Colossal 20 HD Wooden Juggernaut cost 65,000 gp which seems too high (+15,000 for the HD and +50,000 for the two size increases).
Reducing the price of the size increased seems the easiest solution. If we make it +5,000 for the first increase and +10,000 for the second that would make the total +30,000 gp for a Colossal Wooden Juggernaut.
Even that might be a bit too high. It might be easier deciding how much the "Greater Wooden Juggernaut" ought to cost and then back-tracking.
i.e. if the price of a 20 HD wooden juggernaut was, say, 32,000 gp that'd be 17,000 gp more than the base construct price and we could break it down as 600 gp per HD (total +9,000 gp) plus 3,000 gp for Gargantuan size and 5,000 gp for Colossal size.
It'd be a relatively simple matter to tweak those numbers to produce a different final price.
Which reminds me, we should also say what portion of the increased price is increased material cost. Presumably a Colossal wooden construct will require more wood to build than a Huge one.
The easiest option is to just ignore it, but the Greater Stone Golem's cost & price indicate that its materials cost 2,000 gp more than the standard model so I'm thinking maybe we ought to include it. Perhaps 1,000 gp of the Gargantuan size's 3,000 gp is extra material and 2,000 gp of the Colossal size's 5,000 gp?
Alternatively, we could make it some proportion of the price of the HD and/or size increase - maybe 10% of the HD price (so +60 gp of materials per HD) and 20% of the size price (so +600 gp of materials for Gargantuan and +1000 gp of materials for Colossal).
Any preferences?