Data File Growth Afer Restore On 2005

Hi at all,
first post....first help :P .
I hope to help another one (basing on my knowledge) in the next future :)

I'm a beginner dbadmin and I'm looking for help regarding a strange fact relative SQL Server 2005 :)

I migrated a medium(3,5GB) database from sql server 2000 to sql server 2005.
I made a backup in sql server 2000 (it has generated a 3,5GB BAK FILE) and I restored it in sql server 2005 on another server. (I didn't checked data file size).

I configured (with wizard) a maintenance plan on new sql server 2005 with these steps (in this order):
(tellme if the order of the steps is wrong :))

1) check db
2) rebuild index
3) reorganize indexes
4) update statistics (all table and all views)

I planned the maintenance plan on 3a.m. and I went home.

Next day I found that data file (MDF) on sql server 2005 was 22GB large!!
I made a shrink DB but there wasn't free space to erase. It seems there are
22GB of data.
(the data inside is the same of the sql 2000 server..same records same
table...identical, non change of data in the meanwhile)

How is it possible? What am I doing wrong?
I don't understand what can caused the growth of the file, the maintenance plan or the restore? (unfortunately....I didn't checked size after restore...I checked it only the next day).
May be the statistics? (there wasn'int update statistics job on old maintenance plan on sql server 2000)

Thanks in advance :)
Bob