I noticed on my 99 500 cat with stock rims being 12x6.5 rims and 26x12 tires the side walls are sucked in making them way shorter than my 26x9 on 12.6.5 front rims.
And with the width on the mud sharks the case is like 10" with long nobbies sticking out the side making them 12" .
Sure would like to have some 12x8" rims on the back just for that height.

 

I went to the Maxxis web page to check out their specs. On there it gave the actual sizes of the tires. some of their tires actually only measured 9.1" in width but were being classified as a 10" tire. I didn't check for sure but I bet the other tire manufactures have their specs listed at their web sites. Maxxis makes the MUD BUGS, and MUDZILLA's. So if you want to get what you are paying for then this may be a place to check out.