Even though there is a maxWidth prop it is still not easy to make the dialog behave in a specific way. The main problem is it is hard to reach the DialogPaper component. When i style the Dialog component it gets applied to the Modal component instead of the DialogPaper component which makes it difficult to apply a responsive behavior to a Dialog.
The dialogs default behavior is to have the small possible width :-)
You can use the classes.paper property for changing the CSS on that paper element.
It's also related to https://github.com/callemall/material-ui/issues/7448