The clue is if other websites got different font or TMC only. If it is TMC only, then it could be the above.
There is a small magnifying glass in Chrome, snip below, not sure of its name.
That app in the browser, next to yellow bookmark star, acts like zoom or resolution modifier.
Font size can be reset to default by clicking said magnifying glass and clicking reset to default.
That is what happens on my laptop running on Windows 10, but that was happening on W8 as well. It happens on Chrome, not sure of other browsers, but I have not observed it on other browsers. Not sure if that fits with what happens with your font size change.