diff --git a/ocaml/xcp-rrdd/lib/transport/base/rrd_json.ml b/ocaml/xcp-rrdd/lib/transport/base/rrd_json.ml
index b6d1095372..15f95e3de4 100644
--- a/ocaml/xcp-rrdd/lib/transport/base/rrd_json.ml
+++ b/ocaml/xcp-rrdd/lib/transport/base/rrd_json.ml
@@ -45,13 +45,13 @@ let ds_owner x =
   )
 
 let ds_transform x =
-  ( "transform"
-  , match x with
-    | Rrd.Identity ->
-        string "identity"
-    | Rrd.Inverse ->
-        string "inverse"
-  )
+  match x with
+  | Rrd.Identity ->
+      []
+      (* This is the default when transform is absent, and not including it
+         makes the file smaller *)
+  | Rrd.Inverse ->
+      [("transform", string "inverse")]
 
 let bool b = string "%b" b (* Should use `Bool b *)
 
@@ -72,7 +72,7 @@ let ds_to_json (owner, ds) =
          [
            description ds.Ds.ds_description
          ; [ds_owner owner]
-         ; [ds_transform ds.Ds.ds_pdp_transform_function]
+         ; ds_transform ds.Ds.ds_pdp_transform_function
          ; ds_value ds.Ds.ds_value
          ; [ds_type ds.Ds.ds_type]
          ; [