SQL7.0 Backup

I have a database that is set to be backed up(to disk) every night. But as of the past couple of weeks the .bak files are growing at a huge rate, the .bak file is now larger than the acual data being backed up. Was wondering if there is an explantion to this and how to resolve it.
I have checked the “maitenance plan” and made sure that only the specified DB’s are getting backed up(which they are). Some changes that where made when this problem occured was, new replications had been created between several servers. Where i’m knida new to SQL replication and backing up, i’m stuck (and soon my servers will be out of space)
Thanks in advance,
Raoul