How to make a progress-bar thinner?

[Osmo Antero <osmoma gmail com>]

Hello,
How can I make a GtkProgressBar thinner in this application.
See: 
http://www.futuredesktop.com/tmp/test1.c 
And 
http://www.futuredesktop.com/tmp/screenshot1.png

Compile/link the code with 
$ gcc test1.c -o test1 `pkg-config --cflags --libs gtk+-3.0`  
and run 
$ ./test1 

I have tried to use GtkAlignment to set the paddings with 
gtk_alignment_set_padding(GTK_ALIGNMENT(alignment0), 10, 10, 0, 0);
but this then makes the other widgets in hbox0 thicker/fatter. 
Eg. the [Button 1] is fatter than the [Button 2].

This GtkAlignment trick used to work in GTK 2.

The screenshot shows also the actual target application. 
I want to make the level indicator (GtkProgressBar) in the audio-recorder much smaller.

Notice: I have no plans to use gtk-builder or glade.

Do you have any suggestions?

Greetings
 Osmo (Moma) Antero