The horse I know who foundered on grass here wasn't too fat at all... was a Thoroughbred turned out (with other horses who had been there all along and did fine) on a five acre pasture in spring. Foundered within a week. That's the case that made me really paranoid.
Did he suddenly get turned out on it, or had he been there awhile? Suddenly getting turned out on rich spring grass would be the same as getting into the feed bin. If a horse is on pasture as it grows I don't "think" it would be high risk to founder.
Becky