I have found this in the documentation:

-nx number

Size of the nexus in megabytes. The nexus is a contiguous virtual memory area that the engine allocates at startup for caching entity files and level textures and geometry. It speeds up level loading and prevents that the game suddenly aborts at runtime when memory is running low. The nexus size depends on the size of the biggest level of the game. The bigger the nexus, the bigger levels can be rendered - but the more virtual memory is allocated at startup .

When you set the nexus in Map Properties, WED uses the -nx command line option to transfer the nexus size to the engine. The default value for the nexus is 40 megabytes. The maximum value is limited by the Virtual Memory Setting of the Windows OS, minus around 500...1000 MB that should remain free for the operating system. The current nexus requirement is indicated in the statistics panel and can be read from the nexus variable. The recommended maximum nexus value for commercial games is 200. When setting higher values, you should be aware that several Windows subsystems - including DirectX - tend to crash without error message when the virtual memory is running low.

If the nexus size is exceeded, the engine will allocate additional memory from the PC's virtual memory pool. If the virtual memory is also used up, the application will issue an error message and terminate. The level_mark and level_free functions only work when the nexus is not exceeded. In engines older than A7, exceeding the nexus size will produce a "Nexus too small" error message. The engine must then be restarted with a nexus size higher than that you've used before (e.g. -nx 80 for 80 MB nexus size).


So, maybe you can adjust somehow your system settings to achieve a bigger usage. Or not.