[CentOS] lsof with offset

I did something simple; on one window did more/etc/mail/sendmail.cf
on another window did lsof -o -b -p XXX -w 
where XXX is the process number.

The SIZE column does not change to OFFSET!
the size reflects the size of sendmail.cf - I was hoping the "-o" would change 
the SIZE column to OFFSET column and show at what offset the program has read into the file?

AM I doing something wrong?

I am trying to find a way to tell how far a given program has read in a file.

THanks so much.

Jerry