Encryption on duplicate increase the size

Hello,

I activated encryption (256 bits) on my duplicate backups and I noticed that the backups size have increased on the tapes.

I'm not able anymore to backup all my jobs in one tape when the encryption is activated on them.

Is there a way to calculate / anticipate the size that a backup will take with encryption?
Or better, is it possible to decrease the size?

Thanks for your help,

Pierre