In 64-bit LispWorks pointers are 8 bytes, whereas in 32-bit LispWorks pointers are 4 bytes. Since many objects contain pointers, most programs will allocate more in 64-bit LispWorks, typically at least 50% or more.
A program containing mostly strings would not show this difference, since strings are more comparable in size between the two implementations.
You can use find-object-size to find the size of an object.
LispWorks User Guide and Reference Manual - 13 Feb 2015