Comment 6 for bug 332178

Sorry to disappoint you mpt, but I don't think #6 is easily doable here and isn't really worth the effort.

The problem being that GtkButtonBox appears to enforce child widget sizing. This seems more that a little evil to me, and should perhaps be addressed in gtk+. When you insert the buttons, they get added with fill=FALSE, expand=TRUE. It should be a simple matter of using gtk_box_set_child_packing() to set expand=FALSE, but that doesn't work apparently because of this child size_request enforcement.