Starter decant the repitch on yeast?

So, Started making a 2L triple from a single vial of WLP550 for a triple. According to the pitching rate charts, this really isn't enough (BCS- says this is a 3 vial batch). What i haven't seen spelled out is why when I see all the advice on stepping up yeast starter, why the larger volumes each time? is it they actually need the room to grow to larger amounts?

What I'm getting at is what would be wrong with this procedure:
- Wait till fermentation completes
- put the starter in the fridge for a couple days.
- make a new ~1800 ML batch of 1.040 wort in a sauce pan, then chill down to pitching temps.
- While making the wort, decant the spent wort from the starter and let come to room temp.
- Pour new wort on starter "trub"

Q 1) What are the yeasties going to think about that? double again or is their growth stunted by the lack of room?

Q 2) Seperate issue.. with this triple, LHBS says i'm going to need to repitch yeast at bottling. So, I can do dry yeast I guess, but sure would be nice to reuse some of the yeast at the bottom of the fermenter.. if I move the batch to secondary for a month, sounds like i have to do full blown yeast washing to keep that yeast around long enough? And if I just leave it in primary for 3 weeks and skip secondary.. then maybe move the batch to the bottling bucket overnight while making a starter from trub? I get the feeling I'm over thinking this (as I often do)..

Ok, having pondered this I'm going to propose an answer to my own question.. Q1.. with more mouths to feed, need larger volumes of wort (more sugar, without concentrating it above 1.040) for good growth?

I think you're on the right track. Good aeration is also essential for maximum growth, so be sure you aerate the fresh wort as much as possible. I like to put it into something at least twice the size (1 liter of wort in a 2 liter bottle) and shake it like it owes me money. Without dissolved oxygen they will ferment the wort but not really reproduce like you want them to. I also shake/swirl the starter vessel often during the first several hours.

generally the starter volume has to increase because a given wort volume will only 'grow' a certain amount of yeast. If you start with 2 litres and you decant into 2 fresh litres you won't increase the yeast by very much.