diff --git a/src/Lean/Meta/SizeOf.lean b/src/Lean/Meta/SizeOf.lean
index 932f59b65d..449f21da54 100644
--- a/src/Lean/Meta/SizeOf.lean
+++ b/src/Lean/Meta/SizeOf.lean
@@ -149,17 +149,15 @@ partial def mkSizeOfFn (recName : Name) (declName : Name): MetaM Unit := do
           trace[Meta.sizeOf] "declName: {declName}"
           trace[Meta.sizeOf] "type: {sizeOfType}"
           trace[Meta.sizeOf] "val: {sizeOfValue}"
-          -- We expose the `sizeOf` functions so that the `spec` theorems can be publicly `defeq`
-          withExporting do
-            addDecl <| Declaration.defnDecl {
-              name        := declName
-              levelParams := levelParams
-              type        := sizeOfType
-              value       := sizeOfValue
-              safety      := DefinitionSafety.safe
-              hints       := ReducibilityHints.abbrev
-            }
-            enableRealizationsForConst declName
+          addDecl <| Declaration.defnDecl {
+            name        := declName
+            levelParams := levelParams
+            type        := sizeOfType
+            value       := sizeOfValue
+            safety      := DefinitionSafety.safe
+            hints       := ReducibilityHints.abbrev
+          }
+          enableRealizationsForConst declName
 
 /--
   Create `sizeOf` functions for all inductive datatypes in the mutual inductive declaration containing `typeName`
@@ -469,7 +467,6 @@ private def mkSizeOfSpecTheorem (indInfo : InductiveVal) (sizeOfFns : Array Name
         type        := thmType
         value       := thmValue
       }
-      inferDefEqAttr thmName
       simpAttr.add thmName default .global
       grindAttr.add thmName grindAttrStx .global
 
diff --git a/tests/elab/issue5661.lean b/tests/elab/issue5661.lean
index dc2978ca48..8d1afc8359 100644
--- a/tests/elab/issue5661.lean
+++ b/tests/elab/issue5661.lean
@@ -1,5 +1,7 @@
 import Lean.Meta.Basic
 
+set_option warn.sorry false
+
 inductive StructLike α where
   | mk : α → StructLike α
 
@@ -8,7 +10,7 @@ inductive Nested where
   | other
 
 /--
-info: @[defeq] theorem Nested.nest.sizeOf_spec : ∀ (a : StructLike Nested), sizeOf (Nested.nest a) = 1 + sizeOf a :=
+info: theorem Nested.nest.sizeOf_spec : ∀ (a : StructLike Nested), sizeOf (Nested.nest a) = 1 + sizeOf a :=
 fun a => Eq.refl (1 + sizeOf a)
 -/
 #guard_msgs in
diff --git a/tests/elab/issue5661.lean.out.expected b/tests/elab/issue5661.lean.out.expected
deleted file mode 100644
index 50c168d81b..0000000000
--- a/tests/elab/issue5661.lean.out.expected
+++ /dev/null
@@ -1,2 +0,0 @@
-issue5661.lean:48:0-48:8: warning: declaration uses `sorry`
-issue5661.lean:66:8-66:23: warning: declaration uses `sorry`
diff --git a/tests/elab/sizeof1.lean.out.expected b/tests/elab/sizeof1.lean.out.expected
index 2d01b8ea2f..1b2d0726e9 100644
--- a/tests/elab/sizeof1.lean.out.expected
+++ b/tests/elab/sizeof1.lean.out.expected
@@ -1,5 +1,5 @@
-@[defeq] theorem TreePos.leaf.sizeOf_spec.{u, v} : ∀ {α : Type u} {β : Type v} [inst : SizeOf α] [inst_1 : SizeOf β]
-  (a : α), sizeOf (TreePos.leaf a) = 1 + sizeOf a :=
+theorem TreePos.leaf.sizeOf_spec.{u, v} : ∀ {α : Type u} {β : Type v} [inst : SizeOf α] [inst_1 : SizeOf β] (a : α),
+  sizeOf (TreePos.leaf a) = 1 + sizeOf a :=
 fun {α} {β} [SizeOf α] [SizeOf β] a => Eq.refl (1 + sizeOf a)
 theorem TreePos.node.sizeOf_spec.{u, v} : ∀ {α : Type u} {β : Type v} [inst : SizeOf α] [inst_1 : SizeOf β]
   (children : List (List (TreeNeg α β))), sizeOf (TreePos.node children) = 1 + sizeOf children :=
diff --git a/tests/elab/sizeof3.lean.out.expected b/tests/elab/sizeof3.lean.out.expected
index 5a0e05e5c8..e7392d6d04 100644
--- a/tests/elab/sizeof3.lean.out.expected
+++ b/tests/elab/sizeof3.lean.out.expected
@@ -1,11 +1,10 @@
-@[defeq] theorem AList.nil.sizeOf_spec.{u} : ∀ {α β : Type u} [inst : SizeOf α] [inst_1 : SizeOf β],
-  sizeOf AList.nil = 1 :=
+theorem AList.nil.sizeOf_spec.{u} : ∀ {α β : Type u} [inst : SizeOf α] [inst_1 : SizeOf β], sizeOf AList.nil = 1 :=
 fun {α β} [SizeOf α] [SizeOf β] => Eq.refl 1
-@[defeq] theorem AList.cons.sizeOf_spec.{u} : ∀ {α β : Type u} [inst : SizeOf α] [inst_1 : SizeOf β] (a : α)
-  (t : BList α β), sizeOf (AList.cons a t) = 1 + sizeOf a + sizeOf t :=
+theorem AList.cons.sizeOf_spec.{u} : ∀ {α β : Type u} [inst : SizeOf α] [inst_1 : SizeOf β] (a : α) (t : BList α β),
+  sizeOf (AList.cons a t) = 1 + sizeOf a + sizeOf t :=
 fun {α β} [SizeOf α] [SizeOf β] a t => Eq.refl (1 + sizeOf a + sizeOf t)
-@[defeq] theorem BList.cons.sizeOf_spec.{u} : ∀ {α β : Type u} [inst : SizeOf α] [inst_1 : SizeOf β] (b : β)
-  (t : AList α β), sizeOf (BList.cons b t) = 1 + sizeOf b + sizeOf t :=
+theorem BList.cons.sizeOf_spec.{u} : ∀ {α β : Type u} [inst : SizeOf α] [inst_1 : SizeOf β] (b : β) (t : AList α β),
+  sizeOf (BList.cons b t) = 1 + sizeOf b + sizeOf t :=
 fun {α β} [SizeOf α] [SizeOf β] b t => Eq.refl (1 + sizeOf b + sizeOf t)
 theorem Boo.mk.sizeOf_spec.{u} : ∀ {α : Type u} [inst : SizeOf α] (a : α) (cs : BList (Foo α) (Boo α)),
   sizeOf (Boo.mk a cs) = 1 + sizeOf a + sizeOf cs :=