Margins too thin on phone

Hello, I’m a brand new ghost user and I’m find the online editor is not exactly WYSIWYG.

Online, the margins for the “phone” display have the text width extending near the edge of the screen, whereas when I look on my phone at my site, the margins are quite skinny on the post (

Any suggestions for how to resolve this? I’m not sure if the issue is on the ghost / formatting css side, or from my phone (iphone 11 running latest ios)