FWIW, I just checked and the sent mail folder on my iMac, iPhone, and iPad are all identical with two minor exceptions
- The sort order on the iMac was in ascending order and on the two iDevices it was in descending order. That was easily fixed by clicking on "Sort by date" at the top of the list and selecting descending on the drop down menu
- On the iMac I saw all the sent messages from the last 30 days (I have the mailbox preferences set to delete delete sent messsages over 30 days old) while on the iPhone and iPad I had to click on "load more messages" at the bottom of the list to see the entire list. This latter is to save data bandwidth and I don't know that it can be overridden on iOS 5.
So for all intents and purposes the three were identical. If your results are different, I suggest checking the account setup on each device. It may be one of your devices is configured to access the account as POP rather than IMAP or on your iMac check Mail > Preferences > Accounts > Mailbox Behaviors > Sent and be sure the box labeled "Store Sent mail on the server" is checked.