In the developer documentaton for lightning:actionOverride, it states:
In Lightning Experience, the standard Tab and View actions display as a page, while the standard New and Edit actions display in an overlaid panel. When used as action overrides, Lightning components that implement the lightning:actionOverride interface replace the standard behavior completely. However, overridden actions always display as a page, not as a panel. Your component displays without controls, except for the main Lightning Experience navigation bar. Your component is expected to provide a complete user interface for the action, including navigation or actions beyond the navigation bar.
The native new/edit actions appear in a modal, so having the Lightning override take over the full page means that the end user gets a different experience. This is inconsistent UX; the underlying technology should not change the behaviour of the interaction.
With Lightning quick actions, the quick action appears in the modal exactly like other actions. Please make New and Edit override actions behave the same way, so users continue to get a consistent experience in Lightning.