Problem enlarging pages and fonts

I bought all my grandkids a Kobo Clara HD ereader. I used a memory stick to transfer my books to the ereaders. BUT, the kids are not able to make the graphic novels or the book pages get large enough to fill the screen, the pages are small and stuck in a corner of the ereader. What's going on here? Help badly needed.
They can change the fonts of the books that came with the Kobo but not the stuff I have added with the memory stick from my own library.
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