This should work all I did was add a sort to so that your numbers are ordered highest first then set the max results to 5 this way you should only get 5 rows back.
res = service.data().ga().get(ids='ga:' + profile_id, start_date='2014-01-01', end_date=t, metrics='ga:sessions', dimensions='ga:browser',sort='-ga:sessions' , max_results='5' ).execute()
Note: It helps to test your querys you can check the sorting and the numbers that are retruned in the Google Analytics Query Explorer