I am facing the same issue and would also like to know how to solve it, please.
Just like OP, when I try to place keys like large: { ... }, small: { ... }, it doesn’t work as expected. Instead of setting default styles, the entire contents of defaultStyles are placed into the responsiveStyles.large property of the component. This results in large: { large: { ... } ...}.
Is there a way to set default styles for tablet and mobile breakpoints?