Well that’s mainly because they tried to cram everything they possibly could in the huge levels to make it look as good as possible - It’s mainly due to the vegetation. Some maps have hundreds of thousands of vegetation objects (which is admittedly kinda ridiculous, but still works well for the environment)
That’s probably the biggest thing they had to take out for the console re-release, like more then half the vegetation lol.
That’s also why Warhead ran a lot better, it did have some engine optimizations, but for the most part it was the maps. The maps were still fairly expansive, but they didn’t put every single detail they could in it, and there weren’t as many sprawling open areas with vegetation. (In fact, you can get the Crysis levels in warhead and running it in the warhead engine affects performance very slightly, if at all.)