diff --git a/spec/compiler/crystal/tools/hierarchy_spec.cr b/spec/compiler/crystal/tools/hierarchy_spec.cr
index ae1bcd2dbadb..8b971947b299 100644
--- a/spec/compiler/crystal/tools/hierarchy_spec.cr
+++ b/spec/compiler/crystal/tools/hierarchy_spec.cr
@@ -41,6 +41,36 @@ describe Crystal::TextHierarchyPrinter do
                    @x : Bool (1 bytes)\n
     EOS
   end
+
+  it "shows correct size for Proc inside extern struct" do
+    program = semantic(<<-CRYSTAL).program
+      @[Extern]
+      struct Foo
+        @x = uninitialized ->
+      end
+
+      lib Bar
+        struct Foo
+          x : Int32 -> Int32
+        end
+      end
+      CRYSTAL
+
+    output = String.build { |io| Crystal.print_hierarchy(program, io, "Foo", "text") }
+    output.should eq(<<-EOS)
+    - class Object (4 bytes)
+      |
+      +- struct Value (0 bytes)
+         |
+         +- struct Struct (0 bytes)
+            |
+            +- struct Bar::Foo (8 bytes)
+            |      @x : Proc(Int32, Int32) (8 bytes)
+            |
+            +- struct Foo (8 bytes)
+                   @x : Proc(Nil) (8 bytes)\n
+    EOS
+  end
 end
 
 describe Crystal::JSONHierarchyPrinter do
diff --git a/src/compiler/crystal/tools/print_hierarchy.cr b/src/compiler/crystal/tools/print_hierarchy.cr
index b6fcb7a76fdc..0e6b3427020c 100644
--- a/src/compiler/crystal/tools/print_hierarchy.cr
+++ b/src/compiler/crystal/tools/print_hierarchy.cr
@@ -106,8 +106,9 @@ module Crystal
       @llvm_typer.size_of(@llvm_typer.llvm_struct_type(type))
     end
 
-    def ivar_size(ivar)
-      @llvm_typer.size_of(@llvm_typer.llvm_embedded_type(ivar.type))
+    def ivar_size(ivar, extern)
+      llvm_type = extern ? @llvm_typer.llvm_embedded_c_type(ivar.type) : @llvm_typer.llvm_embedded_type(ivar.type)
+      @llvm_typer.size_of(llvm_type)
     end
   end
 
@@ -201,7 +202,7 @@ module Crystal
       max_name_size = instance_vars.max_of &.name.size
 
       max_type_size = typed_instance_vars.max_of?(&.type.to_s.size) || 0
-      max_bytes_size = typed_instance_vars.max_of? { |var| ivar_size(var).to_s.size } || 0
+      max_bytes_size = typed_instance_vars.max_of? { |var| ivar_size(var, type.extern?).to_s.size } || 0
 
       instance_vars.each do |ivar|
         print_indent
@@ -214,7 +215,7 @@ module Crystal
             ivar_type.to_s.ljust(@io, max_type_size)
             with_color.light_gray.surround(@io) do
               @io << " ("
-              ivar_size(ivar).to_s.rjust(@io, max_bytes_size)
+              ivar_size(ivar, type.extern?).to_s.rjust(@io, max_bytes_size)
               @io << " bytes)"
             end
           else
@@ -330,7 +331,7 @@ module Crystal
               @json.object do
                 @json.field "name", instance_var.name.to_s
                 @json.field "type", ivar_type.to_s
-                @json.field "size_in_bytes", ivar_size(instance_var)
+                @json.field "size_in_bytes", ivar_size(instance_var, type.extern?)
               end
             end
           end