The difference is at a given concentration of wort a larger volume of starter has more sugar for the yeast to eat. Volume doesn't correlate to yeast growth, but total amount of sugar does.My understanding is that higher sugar content actually starts the yeast on an anaerobic cycle (like you want in beer) while the lower sugars can still encourage aerobic yeast growth resulting in better cell wall strength and more healthy yeast. The starter is supposed to grow yeast not make beer - yet. I've had great luck with starters down to 1.025.
Maybe tschmidlin can chime in?