On every call to gtk_message_dialog_format_secondary_text, the font size of the primary text label in GtkMessageDialog gets increased, as it multiplies its current size by PANGO_SCALE_LARGE (see setup_primary_label_font). I guess what it really needs to do is multiply by the secondary text font size, rather than its own.

This can be easily replicated with the following code:


#include <gtk/gtk.h>

static void
print_countdown_text (GtkWidget *dialog)
{
	static gint seconds = 10;

	gtk_message_dialog_format_secondary_text (GTK_MESSAGE_DIALOG (dialog),
						  "The application will close in %d seconds", seconds);
	seconds--;
	if (seconds == 0) {
		gtk_main_quit ();
	}
}

static void
timeout_cb (gpointer data)
{
	print_countdown_text (data);
}

int
main (int argc, char *argv[])
{
	GtkWidget *dialog;

	gtk_init (&argc, &argv);

	dialog = gtk_message_dialog_new (NULL,
					 GTK_DIALOG_MODAL,
					 GTK_MESSAGE_QUESTION,
					 GTK_BUTTONS_CLOSE,
					 "Do you want to wait for the program to finish?");
	g_signal_connect (dialog, "response",
			  G_CALLBACK (gtk_main_quit), NULL);
	g_timeout_add (1000, timeout_cb, dialog);

	print_countdown_text (dialog);

	gtk_widget_show (dialog);
	gtk_main ();

	return 0;
}

Attaching a patch to fix it