Links to Safari in Apple Mail opening as half or third of page.

In the past when I clicked onto a link for a Safari page in an email, the page opened up fully and when I wanted to go back to my email, there was a link at the top left of the screen that said Mail. I could click onto that I could go back to my original email. I don't like the Safari page opening to only a small portion of a page because I can't read anything. I've read about Split Screen but I just can't seem to get the Safari Page to open up to a full page no mater how much switching, swiping, touching and tapping I do. Is there a way I can open the Safari link to a full page and then go back to my email when I'm finished reading the link?