KLAX doesn't have anything to to with this, except for the fact that, such a large airport with so many parking spots all very close together (so there's a lot) is able to attract a lot of AIs.
And, HD textures in KLAX DO NOT take more memory than an normal scenery, because they are not used to increase resolution to what it would otherwise be if it used regular textures, but to gain speed, because there are 16x less of them comparing to, say, KDFW, which means the overall memory usage of HD textures is the same as if the scenery used many 1024x1024 textures instead of few 4096x4096.
REX, instead, use 4096x4096 textures for clouds to INCREASE quality so, of course, they take more memory.
Your settings are simply too high, lower them OR use DirectX 10 (KLAX is compatible with it), which takes less memory compared to DirectX 9.