User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-5020152-20121110013131/@comment-5020152-20121110021054

I repeat:

"I think that Fatalis doesn't actively hunt Lao-Shan, instead picking on smaller monsters that have a smaller chance of severely injuring it. Fatalis would only kill and eat a Lao-Shan if it were desperate, had young to feed, or was just feeling aggressive."

There you go.

And even though the size differences, as you say, are large, the difference is still smaller than what I initially thought.