First, have you checked your tire pressures? That has the most effect on gas mileage? Do you know what it was immediately before you traded in your tires, and what it is now -- and did you measure the pressures at the same time of day (e.g., before starting your drive in the morning)? Possibly, your tires were higher before (giving you better mileage). Anyway...

Overall diameter of original size: (195*0.65*2)+(15*25.4) = 634.5 mm
Overall diameter of new tire size: (205*0.65*2)+(15*25.4) = 647.5 mm
New tires are 647.5/634.5 = 1.02, or 2% larger:

Which means that your speedometer underindicates your speed by 2%. This means that when it shows 50 mph (assuming that's accurate), you're really going 51 mph. That's well within the limits of tolerance of a non-certified (i.e., not a police car's) speedometer.
Likewise, when your odometer shows that you've driven 100 miles, you've actually traveled 102 miles.

This is also the equivalent of (in effect) a 2% taller "axle ratio" or perhaps a 2% higher overdrive (it depends on your viewpoint). Usually, taller axle ratios give somewhat better mileage because the engine turns that much more slowly for a given distance of travel. But again, this tiny difference couldn't account for any measureable change in gas mileage.

I doubt that you can actually see, let alone measure, a difference in ride height (as you believe) of half* of the 13 mm difference in diameter, or a mere 6.5 mm (if you're not a "metric" person, this is about 1/4 of an inch).
[ * remember that only the lower half of the tire "lifts" your car's "axles"; the top half of the tire doesn't.]

Of course, we're not counting the additional increase in overall diameter merely due to the difference in tread thickness between a worn tire and a new tire's tread depth. Maybe another 4 or 5 mm or so, depending on how worn your old tires were, but it can't amount to very much.

Of course, a well-worn tire is more flexible than a newer, thicker tread, and could account for some mileage difference: a thin tread's flexing is easier (less rolling resistance), and therefore could have given you better mileage. Differences in sidewall flex is another matter, even more strongly affecting mileage -- but the Defender is a high mileage tire, and may be superior to your old tires in this regard.

All in all, I doubt that changing your tires are completely responsible for all of the 10% loss of gas mileage. In fact, a 10% change is hard to accurately measure, anyway, requiring rigorous protocol (same exact filling point of the tank, same traffic density, same route, etc.). Even differences in the attendant who fills your tank, ambient temperatures, and certainly how much snow is on the ground that you're pushing through, etc., can affect your measurements.