Time Machine Backups fail after restoration

Apparently due to power outages my hard drive became corrupted and I had to erase/reformat it and restore from original 5 year old startup and bundled software discs. Then I restored data from time machine on external networked disk (500 GB) and updated OS to 10.6.8 and updated all software too. Everything seems to have (miracuously!) gone well. Time Machine competed a new backup but now fails saying: This backup is too large for the backup disk. The backup requires 271.78 GB but only 243.71 GB are available. I've excluded apps, caches, downloads, and system files and all the large files but no joy. Should I delete parts of the last backup to make room for incremental backups? Would appreciate any other ideas.