I think you put the scale in the upper left corner of your grid, so it cannot expand lower than the lower edge of button1. Try packing the buttons in a VBox and scale and VBox together in a HBox. btw: for experimenting with layout options use glade.
I got the same result by using Vboxes in a Hbox, but it worked well with Glade! I’m not sure why it worked, but it’s probably some property issues
Thank you!