Font Size Changes when I rotate tablet

I have a page section with a text box and text in standard p1 format. This looks fine on all device sizes. If I view the page on my tablet in Portrait orientation, it is fine. If I rotate it to Landscape it is fine. But, if I then rotate it back to Portrait, the font shrinks dramatically. If I refresh the page it jumps back to the original size. I don't have any custom CSS or anything that should be affecting the font. Examples are attached. The first image shows the page loaded in Portrait mode. The second is after rotating to landscape, and the third is after rotating back to Portrait. Any advise appreciated.

Thanks