Make sure you render in a separate process - if Poser shares it memory with the render engine you will run out pretty quickly in Poser 9.

Texture size is not as important anymore as in the past, but it still takes a large chunk of memory when rendering - another reason to render in a separate process

Morphs take up a whole lot of memory. A single V4 figure grows from 70MB memory to over 300MB if you add/inject all the morphs you have (assuming you have a lot of them)

Set texture preview size (render settings for preview) to 512 or lower. This will free up a considerable lot of memory

If you use displacement - do not use it at high values - this increases memory usage

If you can lower irradiance cache - this takes up a lot of memory as well (during rendering)

For Poser filesize itself does not matter - it is what objects and textures are loaded. If you compress and use PMD a scene can be very small, but still use a lot of memory

 

These are a few things which you  might check