You can try editing assets/js/in5.config.js, but no guarantees as this setting may get applied to multiple aspects of the output.
look for var scaleMode = and set it to
var scaleMode = 'best';
Instead of changing the code (scaling is mainly the JavaScript with transitions in the CSS), try embedding your original output in another document using the in5 Web Content interactive widget and export that new document with 'best fit' applied:
I'm so glad I could help. :)