Firstly, in order to complete a delivery, your recipient’s email server has to have the same capability to receive it. Even if you set your server to have a 20 MB limit, your 15 MB email can’t be delivered if your recipient has a server with a size limit of 10 MB.

Secondly, big size files may crash the email server. Size limits are also needed to avoid malicious users sending very large files in order to crash the server.  They also reduce the amount of disk space needed to store the email messages on the server.

Also, anti-virus software needs to scan the email and a large size file could cause a serious slowdown because of the longer time and higher amount of resources needed to process it.

Lastly, from user point of view, you don't want to download a 30 MB attachment from your email because it can takes long time and might crash some email programs.