diff --git a/src/mongo/db/compact.cpp b/src/mongo/db/compact.cpp
index 32931b6..183e5e9 100644
--- a/src/mongo/db/compact.cpp
+++ b/src/mongo/db/compact.cpp
@@ -114,6 +114,9 @@ namespace mongo {
                         addRecordToRecListInExtent(recNew, loc);
                         memcpy(recNew->data, objOld.objdata(), sz);
 
+                        int sizeChange = recNew->netLength() - recOld->netLength();
+                        getDur().writingInt(d->stats.datasize) += sizeChange; // will usually be negative, ie subtracting
+
                         {
                             // extract keys for all indexes we will be rebuilding
                             for( int x = 0; x < nidx; x++ ) {