Under Settings > Reading, I have set "For each article in a feed show" to Summary, but it's showing the full text. Are you aware of anything in the theme that would override this setting?
You'll need to control the summary from each post, by:
Option 1: adding the "read-more" tag to the content section or
Option 2: adding the summary to The Excerpt section of the post. If you don't see this, you can click the screen options button at the top right and make sure it's checked.
Option 3: you can use something like Advanced Excerpt to control this on a more global level.
Let me know if that helps