Messages of different sizes. Messages with attachments will be the biggest.
We said earlier that your mailbox size is determined by the size of its folders. Logically enough, a folder's size is determined by the sizes of the messages (and calendar appointments, contact information, notes, and so on) contained within it. Some specific attributes such as fancy formatting, pictures, and attachments can make messages bigger.
Average sizes for messages will vary from organization to organization and from person to person. A very rough estimate is that an average-size message is about 30 KB. And, like mailbox size, message size limitations are set by a system administrator at the server level — they're not something you can change in Outlook.
So now that you know a bit about size and what makes your mailbox big, let's talk about some quick ways to trim it.