This is likely because your menu isn't made responsive.
It's not surprising given how you created the menu. Also there is nothing you can do about end user preferences.
A much better question is WHY are you STILL using discontinued Muse for new projects?
No one wants to increase the screensize or the size of a browser window.
Visually impaired persons use a screenreader, that´s why webfonts are so important and image descriptions and so on, so screenreaders are able to work properly. If you want to encrease fonts, you might be better with either WP or Joomla and provide a button to encrease text only and not encrease the complete browserwindow which is quite unprofessional, and although this could be done with Muse as well it is of course not very handsome because content must be doubled or even tripled in Muse to achieve that - noone wants that. And so again, no one wants to encrease the browser window by CMD+ or STRG+.
And responsive webdesign was not meant for that but you know this, I hope.
There is s term called "accessible websites" - that is the way to follow - …
I was clinically blind before my eye surgery --20/1440. I never used a screen reader because I could see just not very well. I did what most vision impaired people do, increased the default font sizes in my browser settings.