On the iPhone when you are looking at a mail it shows a preview of the email. You can edit the number of lines that are shown (to a point) or turn them off entirely to make the list smaller. To edit this go to home screen and select "Settings" and then select "Mail". In the "Messages" section of the mail options, click on "Preview" and it'll let you choose from 0 to 5 lines as the length of the email preview.