To reduce the size of the whole image, use
clean-down.
In 32-bit LispWorks, you can use
(clean-down)
or the less aggressive
(clean-down nil)
to reduce the image size when the image is much larger than the amount that is allocated. In 64-bit LispWorks there is no need to do that.
LispWorks User Guide and Reference Manual - 21 Dec 2011