You cannot zoom in in the app (at least as of Garmin Connect Mobile for Android v3.6.0.1), but you can tap on a HR, Cadence, Pace or Elevation chart for an activity to enlarge it by bringing it up in landscape orientation on your mobile device. If that isn't sufficient, then your best bet is to bring up the activity in Garmin Connect online on a computer and click on the icon to zoom in.