If your email client is configured to use POP, it could be conifgured to leave a copy of each message the server for a number of days after you have already downloaded the message onto your computer. This setting can be helpful if you want to check mail from multiple devices (such as a computer and a mobile phone), so many email programs are configured by default to leave mail on the server for 14 days.
That said, the length of time that messages are stored on the server has a direct impact on the amount of storage being utilized for your email account on the server. This can quickly magnify if you frequently receive large email attachments.
One way to reduce the amount of storage being utilized on the server is configure your email program to leave mail on server for a smaller number if days, or to disable leaving mail on the server entirely if that feature is not needed. Mail can still be retained on your computer for as long as you desire.