font-size is inherited from the same parent for the elements in question. however it displays differently. I am at complete loss on how to solve it.
But if I let max-width of the container element be 100% on vertical verwion of website, the text grows despite font-size not changing.
Presumably, your pages lack the viewport
meta instruction, like for example
<meta name="viewport" content="width=device-width, initial-scale=1">
so their scaling is completely up to each device: some may render it in a fixed default ~800px wide virtual viewport and then they may or may not try to scale that down into the actual viewport (what corresponds with your screenshots, I guess). Adding aforementioned meta should usually help.