RadImageGallery fullscreen mode overrides viewport meta tag set on page.
If the user sets the following meta tag:

<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1, maximum-scale=1.0, user-scalable=no, height=device-height" />

initially the page is not scalable. When the Gallery enters fullscreen mode, the page is zoomed out.
And when you exit fullscreen, the page stays zoomed out, the gallery proportions are changed and fonts are smaller.