I get the insurance policy aspect of a starter - no question there. assuming everything is fine though I don't see how the yeast "knows" it's in a 2L starter vs a full on batch of beer. and with a vitality starter I'm pitching everything in so any byproducts the yeast produce while "waking up" in the starter are going into my batch of beer anyway.
What I do for wakeup starters (I use a stirplate) is, once the yeast is woken up and the starter wort is depleted, I crash-cool the starter for 24-48 hours before brewday, to let the yeast settle, then decant the spent wort off the yeast cake, In my experience, this helps limit any potential impact due to byproducts of the starter itself.
I can't comment on the effect of pitching the entire volume of wort from a starter on flavor since that's not part of my brew process.
I agree that the yeast has no idea what volume of wort it's being pitched into, but that's not really the issue at hand; making sure it's in prime fighting shape is, hence the idea of a vitality starter.
If you're making a vitality starter, you have the chance to see exactly how much slurry you're adding to the starter wort, so you have a better grasp of how much yeast is being pitched on brewday, so you can ensure you're not under-pitching.