OpenEXR supports tiled mip-maps - JPEG doesn't do that. Also, JPEG is a lossy format, where the EXR compression that Poser uses is lossless.

Tiled file formats allow a program to read just a section of it without having to read the entire file into memory. Poser leverages that feature in order to reduce the memory required when rendering: instead of having to read entire texture maps, the renderer can read only the sections it currently needs.