Migration Assistant report insufficient storage for Time Machine restore on the same computer

Erased my 2019 macbook pro to do a fresh install from time machine. Migration assistant says there is not enough room to do a restore from a time machine backup. It says only 100gb available on a 1tb hard drive. Is there anyway to do a backup from the time machine drive?

[Re-Titled by Moderator]

Original Title: Why does migration assistant say not enough storage to back up time machine restore onto the same computer it's backed up from?