according to test i pratice (very few ), it seems that the "standard" backup have a smaller size and works on backgroud whereas the nbackup seems modal ?
Yes, IBCBackupService backs a database up on the host machine. This behaviour is connected with the specificity of InterBase server, and not with IBDAC.
Sizes of the backup files are different because the files have different format. You can learn more about this in the InterBase server documentation.