As I recall there was an article on Drovers, perhaps by Kris Ringwall of NDSU on this smaller cow vs larger cow topic not too long ago. That would have some data.

However I think we can all agree that a smaller 1300 lb cow is much more capable of weaning a 50% 205 day 650 lb calf than a larger cow is likely to wean a 900 lb 50% 205 day calf. The 1800 lb cow is probably going to wean about a 650 lb 205 day calf also and likely going to require more feed to do it over those 205 days. All no creep.

How much more feed is used by the 1800 lb cow probably varies with a lot of different factors.  But the basic direction of a smaller cow being more efficient in use of feed/forage seems pretty clear.