Crocodilians, for example, seem to have a distribution around a higher mean than, say, birds or mammals. Body size is, I believe, taxon-dependent. Let's check this. But I'm not sure about this point. And yet mammals are bigger.
Beaks are awesome tools.
I have trouble believing that eating competitions result in either large mammal preeminence, or the greater diversity in small birds. From what I've read I think you would agree with this. Like you I would be interested in hearing how any competitor in this respect could affect ostrich distribution. Indeed, ostrich adults living among the world's most formidable carnivores suffer little predation. Winged flight enables a bird to forage and then escape to cover.