I think activity focused on topic uses the default way a topic list looks, which is usually the title, but themes and theme components can add an excerpt. Activity focused on topic is, for example, also used for the voting activity /my/activity/votes. It allows a quicker overview of which topics you voted for, and the titles usually work as a good summary.
For activity focused on individual posts, showing only the topic title wouldn’t help much. Whether it’s about the user having posted something or liked something, you need information about the post itself, not only the topic title. That’s why showing some content is needed here.
