When growing large (more than the initially reserved 0.5GB) LispWorks 5.1 (32-bit) for Windows "skips" over address space occupied by DLLs which it cannot use. This allows it to grow above the libraries and thus achieve a larger maximum heap. The size depends on the mapping of the libraries.
LispWorks 5.0 (32-bit) for Windows and previous versions requires a contiguous heap and thus its heap is limited in practise to less than 1GB.
For more information about the memory layout of LispWorks see "Startup relocation" in the LispWorks User Guide .