Just because it says 'full size spare' doesn't mean you should expect a 18" rim down there...
You have your Txxx/xx/Rxx wheels which are very temporary.
You then have your full size spares which may or may not actually be the same rim diameter.
What it usually means, in the case of the Focus, is that you have 17" rims but with thicker tires so that the outer diameter of the spare matches the outer diameter of the remaining stock wheels so you don't run into shenanigans, this is especially important for AWD equipped vehicles because not having matching tires or matching treadwear can lead to premature expiration of one or more of the tires.
Even my truck, with 255/45/R20 rubber only had a 18" spare, the spare made up in the difference by having much thicker tires.
2012 Ford Focus Titanium Blue Candy Hatch 2009 Suzuki Swift+ - Traded in 2006 Lincoln Zephyr - Traded in
If you live in Edmonton, AB I would be more then happy to do a shoot with you!