Oracle Database Discussions

I have a table with a lob segments (about 250Gb) (Enterprise edition 12c)

I create a new partitioned empty table than I used the statement:

insert /* append parallel(4) */ into new_table  select * from old_table

The new table size is less than 50Gb.

I checked the number of row and it matches.

How it's possible? Fragmentation?

And if It's Fragmentation, how can I know, in advance, how much space an operation like this can save?

I mean some query that I can run against the old table for check space that can be released.

Thanks in advance.