Charlene and everyone else writes that index size doesn't matter ... it is all relevancy.
actually, i think it matters a lot. a real lot. relevancy is very important, of course ... but relevancy comes after size ...
let me explain ...
Doing a quick search for my name ("Auren Hoffman"), i get 7,160 results in Google and 11,800 results in Yahoo. (but who knows if it is true as they each only show the first 1000 results) when you search for something more specific [Auren Hoffman sv100] you get the same number of results in both.
but ... and here is the but ... not long ago i never got comparable results in Yahoo. Google used to dwarf Yahoo by 2x or more. so i always assumed that Yahoo was missing docs that Google didn't have. so even if yahoo claimed it had better relevancy, i wasn't going to switch from Google.
now that i am satisfied that Yahoo has indexed about the same number of pages as Google, i might actually consider switching to Yahoo.
so size is really important ... because i'm not even going to consider a better relevancy engine if it only indexes 50% of what Google has.