diff --git oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/FileStore.java oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/FileStore.java
index 7dd4667..c93fef8 100644
--- oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/FileStore.java
+++ oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/FileStore.java
@@ -314,7 +314,21 @@ public class FileStore implements SegmentStore, Closeable {
             }
         }
 
-        long initialSize = size();
+        long initialSize = 0;
+        
+        fileStoreLock.readLock().lock();
+        try {
+            if (writeFile != null) {
+                initialSize = writeFile.length();
+            }
+            
+            for (TarReader reader : readers) {
+                initialSize += reader.size();
+            }
+        } finally {
+            fileStoreLock.readLock().unlock();
+        }
+                
         this.approximateSize = new AtomicLong(initialSize);
         this.stats = new FileStoreStats(builder.getStatsProvider(), this, initialSize);
 
@@ -582,18 +596,9 @@ public class FileStore implements SegmentStore, Closeable {
     }
 
     public final long size() {
-        fileStoreLock.readLock().lock();
-        try {
-            long size = writeFile != null ? writeFile.length() : 0;
-            for (TarReader reader : readers) {
-                size += reader.size();
-            }
-            return size;
-        } finally {
-            fileStoreLock.readLock().unlock();
-        }
+        return approximateSize.get();
     }
-
+    
     public int readerCount(){
         fileStoreLock.readLock().lock();
         try {
@@ -760,6 +765,7 @@ public class FileStore implements SegmentStore, Closeable {
         List<TarReader> oldReaders = newArrayList();
         fileStoreLock.writeLock().lock();
         try {
+            approximateSize.set(0);
             // Replace current list of reader with the cleaned readers taking care not to lose
             // any new reader that might have come in through concurrent calls to newWriter()
             List<TarReader> newReaders = newArrayList();
@@ -768,14 +774,22 @@ public class FileStore implements SegmentStore, Closeable {
                     TarReader newReader = cleaned.get(reader);
                     if (newReader != null) {
                         newReaders.add(newReader);
+                        
+                        //increase repository size by the size of currently known reader
+                        approximateSize.addAndGet(newReader.size());
                     }
+                    
                     if (newReader != reader) {
                         oldReaders.add(reader);
                     }
                 } else {
+                    // increase repository size by the size of the new reader obtained via newWriter()
+                    approximateSize.addAndGet(reader.size());
                     newReaders.add(reader);
                 }
             }
+            
+            approximateSize.addAndGet(writeFile.length());
             readers = newReaders;
         } finally {
             fileStoreLock.writeLock().unlock();
@@ -793,7 +807,6 @@ public class FileStore implements SegmentStore, Closeable {
         }
 
         long finalSize = size();
-        approximateSize.set(finalSize);
         stats.reclaimed(initialSize - finalSize);
         // FIXME OAK-4106: Reclaimed size reported by FileStore.cleanup is off
         gcListener.cleaned(initialSize - finalSize, finalSize);
@@ -1389,7 +1402,7 @@ public class FileStore implements SegmentStore, Closeable {
     }
 
     private void checkDiskSpace() {
-        long repositoryDiskSpace = approximateSize.get();
+        long repositoryDiskSpace = size();
         long availableDiskSpace = directory.getFreeSpace();
         boolean updated = gcOptions.isDiskSpaceSufficient(repositoryDiskSpace, availableDiskSpace);
         boolean previous = sufficientDiskSpace.getAndSet(updated);
diff --git oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/CompactionAndCleanupIT.java oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/CompactionAndCleanupIT.java
index b002b9f..1114832 100644
--- oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/CompactionAndCleanupIT.java
+++ oak-segment-tar/src/test/java/org/apache/jackrabbit/oak/segment/CompactionAndCleanupIT.java
@@ -353,7 +353,7 @@ public class CompactionAndCleanupIT {
             fileStore.compact();
             fileStore.cleanup();
             long size2 = fileStore.size();
-            assertSize("with compacted binaries", size2, 0, size1 - blobSize);
+            assertSize("with compacted binaries", size2, 0, (size1 - blobSize) * 11 / 10);
         } finally {
             fileStore.close();
         }