Well, let's think of it this way (all calculations are based on cm2 for the unit of area). The area of an 8" mirror is roughly 324 cm2 and that of a 10" mirror is roughly 507 cm2. The difference in area is then 507 cm2 - 324 cm2 = 183 cm2. Now if you compare that value to the surface area of the 8", 183 cm2 ends up being about 56% the size of that area, so a 56.25% increase in surface area from 8" to 10" doesn't sound so weird after all.
Or perhaps I've just confused everyone (and even myself!) just a little more!
