@MaximeDemers -- Unfortunately, iframes are not inherently responsive, so content embedded within them doesn't know how to behave. In this case, the embedded story isn't getting any information about the size of the frame, so it's not able to adjust its layout like you see when opening the story from the direct URL.
You'll need to add some CSS and/or JavaScript to your page to ensure you get the experience you are looking for when embedding using an iframe. There are many articles on the web with more guidance about this if you search "responsive iframe."
You may be interested in learning more about our new✨ advanced embedding capability ✨. Read more about it here: Introducing Story Embeds via Script!
Owen Evans
Lead Product Engineer | StoryMaps
