Hello,
The Mega menu dropdown width is set according to the elements in the categories menu container in the header like browse categories text and icons etc.
Screenshot for Clarification: https://ibb.co/dmpDKXr
If you want to change the width of the dropdown to a specific point then you need to use a custom CSS for that.
If you want to ask anything else then provide with some relevant screenshot of the specific area which you want the width to be according to the content to check it myself and help you out accordingly.
Regard’s.
Xtemos Studio.