Yes the new UI supports large buttons only, so the options to turn on small buttons are disabled.

We did this because the new flyout bars allow us to provide more buttons without having to fit them all on a permanently visible bar. And because it means we don't need to double up every icon design and on the default bar layouts by having both large and small versions (the 16x15 sizes were a real challenge for the icon designers to make them recognisable). It also allows us to make the different related products (Web Designer, PGD, Pro, etc.) have similar bar layouts which helps with help text and screenshots many of which are shared between the different products.

The old UI which you can switch to (as mentioned elsewhere) does still support large and small icons, but in future releases this may not be supported.

Neil