Windows 3.x guests are typically limited to 64 MB RAM, even if a VM is assigned much more memory. While Windows 3.1 is theoretically capable of using up to 512 MB RAM, it only uses memory available through the XMS interface. Versions of HIMEM.SYS, the Microsoft XMS manager, shipped with MS-DOS and Microsoft Windows 3.x can only use up to 64 MB on standard PCs.

This is a known HIMEM.SYS limitation. Windows 3.1 memory limits are described in detail in Microsoft Knowledge base article KB 84388.

It is possible for Windows 3.x guests to utilize more than 64 MB RAM if a different XMS provider is used. That could be a newer HIMEM.SYS version, such as that shipped with Windows 98, or a more capable third-party memory manager, such as QEMM.