Glib's source contains this line:

    return g_strdup_printf (g_dngettext(GETTEXT_PACKAGE, "%u byte", "%u bytes",(guint) size), (guint) size);
 (#: glib/gfileutils.c:1739)

The strings here are not extracted by damned-lies[1], only running make glib20.pot extracts these. Makefile.in.in contains this: 

	  --keyword=g_dngettext:2,3 \

But intltool-update seems to ignore this, so it seems that this is really an intltool bug, but given the short time until 2.28, would it be possible to circuminvent somehow this limitation in d-l?

This string is quite visible, one can see as the size of files under 1 KB.

[1]: http://l10n.gnome.org/POT/glib.master/glib.master.pot