Leosghost, are you talking about Google caching SERPs? So if someone searches "blue widgets" he/she gets (essentially) a cached results set that is then passed through a personalisation filter (personalised results) - database untouched. Perhaps this is what you are you saying when you mention "pre-sorting" (caching?). If so, I'm very sure I've heard MC say this is the case in one of his videos - Google cache results, hence why you get such fast results. Those caches probably update on a schedule and this is where true "heavy lifting" processing happens (generation of caches) - and the new caches are echoed across the data centers. Google get new queries everyday of course, and I'm sure they try to find the nearest used query cache to retrieve for that new query (and may even create a queue of "new queries" from which the algo needs to generate corresponding caches for).
All of the above is speculation by me :)
Perhaps the index is - as you say - getting too unweildly - and so they're making quite brutal decisions on separating wheat from the "chaff", with the results all to clear to a lot of us - some chaff remains, a fair amount of wheat thrown away - but all expedient to Google's processing goals.
Ultimately though - and this is what REALLY matters to us - is Google simply becoming an ever-losing proposition to small site owners? If so, there's still a demand for small businesses and the services / products they provide. If Google won't supply that demand, then someone has to.