: Anyways, could it be draw distance? I have noticed that the Mammoth suffers
: from significantly less pop-in. IIRC

I have no idea about the mammoth, but if it's anywhere in the realm of the scorpion (I imagine even moreso), then it's probably a massive memory drain that would suck resources from other parts of the level. For example, in Halo Reach, "The Package", the scorpion's presence in the level meant we could not load in the warthog, phantom or pelican vehicles. Even though they were planned to be in the level. The scorpion was originally planned to be air dropped by a pelican but had to be changed due to memory constraints. Even the Brutes were removed from those spaces due to memory limits.

Simply put, the larger, more detail heavy characters and vehicles take up more memory and must be juggled around to make sure everything fits.