diff --git a/releng/README b/releng/README
index 2d3054d1..d214efe4 100644
--- a/releng/README
+++ b/releng/README
@@ -124,7 +124,8 @@ the user or require sysadmin considerations before they are run:
   Exercises the production of a bootable Debian GNU/Linux image and its Jigdo
   files. This test downloads a Debian daily image for i386 of about 270 MB,
   extracts its content and composes a new image. Thus it needs about 850 MB
-  of disk space in releng/releng_generated_data.
+  of disk space in releng/releng_generated_data when unpacked. Adding the daily
+  image size itself, the total space used would peak at about 1.2 GB.
   This test will only work with GNU xorriso or if libjte was installed already
   when libisofs was built. Further it needs the program jigit-mkimage. Both
   are part of package jigit, version >= 1.18, available at: