Chakra-UI: How to apply size to Heading using chakra factory function

I want

<SidebarHeading>heading</SidebarHeading>

to be a synonym for

<Heading mt={5} size={'sm'}>heading</Heading>

I tried

const SidebarHeading chakra(Heading, { baseStyle: { mt: 5, size: 'sm' } })

This applies the top margin but not the size. How do I specify the size?

Is the second parameter of chakra() documented somewhere? I've only found a set of usage examples.