ComboBox in Backstage

I am not having any luck getting ribbon:ComboBox to look right in Backstage. I am using the settings Context=BackstageItem and VariantSize=Small, but I could change or remove those settings altogether, and I still get the same result - a really big and tall combobox that does not look like the ones I find in Microsoft office products. What am I missing?

Thanks in advance for your help!