I run a 22 tooth granny with a 36-tooth big cog in back, but even so I know from experience that four or five miles of unremitting 7 - 9 % grades on a 75 lb bicycle will have me wishing for another 2.8%. I guess I'm not a very strong rider.

But more than simple ratios, there's gotta be complex issues of internal musculature and anatomy involved, in my case climbing with 175mm cranks just feels easier, and more than 2.8% easier.