Hi there, my first post here, I think. I randomly came across something that I think might help the issue with large animals starving.
http://en.wikipedia.org/wiki/Kleiber%27s_lawSimply put, for the vast majority of animals in the real world, the required caloric intake only goes up 75% for every double in size. Therefore, a 12,800 lb. elephant only has to intake 29 times the calories as a 200 lb. deer, despite being 64 times larger.
Now, I'm looking at the Pasture page on magmawiki and it looks like there is a linear progression for how much an animal has to eat, to the current detrimental effects. I see how every animal's Grazer Amount * Creature Size = ~60,000,000. According to Kleiber's law, that shouldn't be the case. It looks like like it's more than 8 times harder to feed an elephant as it is a cow. It should really only be ~5 times harder. But, why start with the cow? A cow is currently 2 times harder to feed than a donkey (the cow being twice its size) when the cow should only be eating 175% what a donkey does.
I realize Toady probably already has heard the suggestion that larger animals have a lower relative metabolism, but it seems like he likes to be precise about things. My guess is to figure out what animal has the most desirable in-game grazing, then scale that up or down by 175% for each double/half in body size (i.e. if the donkey is the best at Grazer Amount 200, then the elephant should be ~21; if you go off of alpaca, then elephant should be ~30; and, basing off of groundhog would set elephants at ~130). That'll do for rough guestimation of values in between, but I'm sure Toady understands the math on the wiki page to make use of it.
I hope this can help!