This is exactly what prathmann was saying above, but it's a pet peeve of mine so here I go.... I think what makes this so confusing is that the stated size (26", 29", 700c, 27") are the estimated diameter including the tire, but people use the numbers for "rim size". It is not the rim size. So even though the 700c and 29" use the same rim diameter (622mm ~ 24.5"), the 29'er generally uses a larger tire and therefore has a total size that is larger than the 700c (700 mm ~ 27.5").

And of course, ultimately the actual total diameter depends on the size of the tire itself.

This is just stupid and I can't for the life of me understand why companies and people don't just use the actual rim diameter. Of course this info is printed on every tire and tube you can buy, so why don't we use it.