Fixed width formats are only efficient when everything in a column is a similar length. You only need a single 1000 char data value in a column to mean either the whole column is 1000 chars or the data is truncated.
Fixed width formats are pretty efficient for quick random access.
Btw, you can have a mostly fixed width format, but with an escape that puts larger value somewhere else. That's more or less what some relational databases are doing, I think.
