diff --git a/xpcom/base/nsMemoryReporterManager.cpp b/xpcom/base/nsMemoryReporterManager.cpp
index c29bb82d0c3..41d5ad0225f 100644
--- a/xpcom/base/nsMemoryReporterManager.cpp
+++ b/xpcom/base/nsMemoryReporterManager.cpp
@@ -141,7 +141,7 @@ static PRInt64 GetVsize()
   if (!success)
     return -1;
 
-  return s.ullTotalPhys - s.ullAvailPhys;
+  return s.ullTotalVirtual - s.ullAvailVirtual;
 }
 
 #if MOZ_WINSDK_TARGETVER >= MOZ_NTDDI_LONGHORN