What does "d+w user bss" mean and is there a parameter that controls this value?

We're trying to reduce the memory footprint of a couple of our landscapes, and we found that the "d+w user bss" area takes up over 80% of the memory used in each work process. Why is it such a large part of each work process? Is there any way to reduce the size of this area, by a user parameter or otherwise?

Thanks very much.

Gordon