In Sharepoint 2003 there is silly hard-coded limitation while you wish to save site as template include the content. The template is limited to 10MB!!!
In MOSS 2007 you can change that limit using stsadm execute the following command :
stsadm -o setproperty -pn max-template-document-size -pv 40000000
(where max-template-document-size is in Bytes)