Webmin takes memory readings from top VIRT (Virtual) and is almost always noticeably bigger than RES (Resident). Webmin is reporting the virt value which includes memory shared with other processes (library code that is loaded into real memory only once but mapped into the virtual address spaces of multiple processes. i.e Java) and memory that is allocated but not yet used (it is available in the processes virtual address space, but no real RAM or swap pages have been allocated for it yet).