Offsite backups are designed to intelligently backup only those files that changed since the last backup. That means the first backup is big, but every day after that should be much smaller.
Total offsite backup storage is calculated by adding together the space used by the first full backup with the 30 days of smaller incremental/daily backups.
Each server subscription includes a certain amount of offsite space. If your storage requirements exceed the included space, a fee of $3/GB per month will be applied.