Yes, it is by design. I think.
Smaller sizes don't make too much sense when there are other controls (like combo and choice boxes) in the toolbars.
The GUI toolkit sizes these based on the font and the size of the toolbar is still as big as if you've used the native sizes, but now the images are smaller and it looks ugly anyway and no space is save (at least vertical).
p.s. If you're willing to experiment with this patches welcome.
