I think John's right that it does/probably/maybe (pick your preferred term) fit, but that it does so in a less-than-ideal way -- secure, but visible and/or a tight fit.
I checked Mercury Mariner Hybrid on Google and it looks like it only has the option of the small/temporary spare tire, just like the Ford Escape Hybrid.
I didn't check the non-hybrid Escape, but during my search the non-hybrid Mercury Mariner came up and it does have the option of a full-size spare ($105 extra in '06 model), so that seems to lend credence to the "because of the hybrid components" (whether that's the battery, coolant lines for the battery's A/C system, or what, I don't know), a full-size spare mounting is sub-optimal in some way.
I'd checked out my spare on Easter but did not go to the trouble of removing a perfectly good stock/factory tire from my vehicle to test fit a full-size "spare" where the small/temporary spare lives. I just cranked my spare down while familiarizing myself with its location, ease of access, etc.
Like I said in another forum, I drove 14 years in my Corolla and the one and only time I needed my full-size spare (or any spare) tire, I found my spare tire was flat and still had to call roadside assistance. Murphy's Law...
