diff --git a/internal/backends/collection_test.go b/internal/backends/collection_test.go
index ce984a466277..206fd27fbfbc 100644
--- a/internal/backends/collection_test.go
+++ b/internal/backends/collection_test.go
@@ -189,19 +189,20 @@ func TestCollectionStats(t *testing.T) {
 				db, err := b.Database(dbName)
 				require.NoError(t, err)
 
+				var c backends.Collection
 				cNames := []string{"collectionOne", "collectionTwo"}
 				for _, cName := range cNames {
 					err = db.CreateCollection(ctx, &backends.CreateCollectionParams{Name: cName})
 					require.NoError(t, err)
-				}
 
-				c, err := db.Collection(cNames[0])
-				require.NoError(t, err)
+					c, err = db.Collection(cName)
+					require.NoError(t, err)
 
-				_, err = c.InsertAll(ctx, &backends.InsertAllParams{
-					Docs: []*types.Document{must.NotFail(types.NewDocument("_id", types.NewObjectID()))},
-				})
-				require.NoError(t, err)
+					_, err = c.InsertAll(ctx, &backends.InsertAllParams{
+						Docs: []*types.Document{must.NotFail(types.NewDocument("_id", types.NewObjectID()))},
+					})
+					require.NoError(t, err)
+				}
 
 				dbStatsRes, err := db.Stats(ctx, new(backends.DatabaseStatsParams))
 				require.NoError(t, err)
diff --git a/internal/backends/postgresql/database_test.go b/internal/backends/postgresql/database_test.go
index 3ca5a4c3fe07..5b3d8f9c173f 100644
--- a/internal/backends/postgresql/database_test.go
+++ b/internal/backends/postgresql/database_test.go
@@ -69,7 +69,7 @@ func TestDatabaseStats(t *testing.T) {
 		require.NoError(t, err)
 		require.NotZero(t, res.SizeTotal)
 		require.Equal(t, res.CountCollections, int64(len(cNames)))
-		require.NotZero(t, res.SizeCollections)
+		require.Zero(t, res.SizeCollections)
 		require.Zero(t, res.CountObjects)
 		require.NotZero(t, res.CountIndexes)
 		require.NotZero(t, res.SizeIndexes) // includes metadata table's indexes
diff --git a/internal/backends/postgresql/postgresql.go b/internal/backends/postgresql/postgresql.go
index 4a7d54b37e01..af4c465f04e2 100644
--- a/internal/backends/postgresql/postgresql.go
+++ b/internal/backends/postgresql/postgresql.go
@@ -67,10 +67,23 @@ func collectionsStats(ctx context.Context, p *pgxpool.Pool, dbName string, list
 		args = append(args, c.TableName)
 	}
 
+	// The table size is the size used by collection objects. It excludes visibility map,
+	// initialization fork, free space map and TOAST. The `main` `pg_relation_size` is
+	// used, however it is not updated immediately after operation such as DELETE
+	// unless VACUUM is called, ANALYZE does not update pg_relation_size in this case.
+	//
+	// The smallest difference in size that `pg_relation_size` reports appears to be 8KB.
+	// Because of that inserting or deleting a single small object may not change the size.
+	//
+	// See also https://www.postgresql.org/docs/current/functions-admin.html#FUNCTIONS-ADMIN-DBSIZE,
+	// visibility map https://www.postgresql.org/docs/current/storage-vm.html,
+	// initialization fork https://www.postgresql.org/docs/current/storage-init.html,
+	// free space map https://www.postgresql.org/docs/current/storage-fsm.html and
+	// TOAST https://www.postgresql.org/docs/current/storage-toast.html.
 	q = fmt.Sprintf(`
 		SELECT
 			COALESCE(SUM(c.reltuples), 0),
-			COALESCE(SUM(pg_table_size(c.oid)), 0),
+			COALESCE(SUM(pg_relation_size(c.oid,'main')), 0),
 			COALESCE(SUM(pg_indexes_size(c.oid)), 0)
 		FROM pg_tables AS t
 			LEFT JOIN pg_class AS c ON c.relname = t.tablename AND c.relnamespace = quote_ident(t.schemaname)::regnamespace
diff --git a/internal/backends/sqlite/sqlite.go b/internal/backends/sqlite/sqlite.go
index 45e7e4d32da0..473f19a19440 100644
--- a/internal/backends/sqlite/sqlite.go
+++ b/internal/backends/sqlite/sqlite.go
@@ -70,6 +70,16 @@ func collectionsStats(ctx context.Context, db *fsql.DB, list []*metadata.Collect
 		indexes += int64(len(c.Settings.Indexes))
 	}
 
+	// The table size is the size used by collection objects. The `pgsize` of `dbstat`
+	// table does not include freelist pages, pointer-map pages, and the lock page.
+	//
+	// If rows are deleted from a page but there are other rows on that same page,
+	// the page won't be moved to freelist pages.
+	//
+	// The smallest difference in size that `pgsize` reports appears to be 4KB.
+	// Because of that inserting or deleting a single small object may not change the size.
+	//
+	// See https://www.sqlite.org/dbstat.html and https://www.sqlite.org/fileformat.html.
 	q = fmt.Sprintf(`
 		SELECT SUM(pgsize)
 		FROM dbstat