Are you using the default font and size? If so then I think the report you get is possibly (one of many) false positives.
I don't think it's normally necessary for a site to need a larger font size on mobile, but if you are using a different font then it may be necessary to go up a pixel overall.
This happened to me when I switched from the default to Raleway font, people (users) complained that 15px was too small both on desktop and mobile. But the default 15px is fine with the default font/s