Thank you for your question.
- your store URL;
- page URL with the issue you mention;
- storefront password (if your store has one).
Please keep in mind that whenever you request technical assistance we are unable to help without having your store url.
I'm afraid this is a much more complex issue then it meets the eyes. To put it as succinctly as I can, what is limiting the dropdown to be full width is the navigation's wrapper. The green parts are the invisible limits of it.
Nothing within it can expand that green limit.
We'd need to restructure your navbar to be able to be expanded (basically remove the green bars) and then add CSS to make it look like it already is looking. It'd require some work.