I would say do a factory reset and you'd probably be ok. What were you trying to get from transferring from your iPhone? Just asking because there is probably other ways to accomplish that without doing the switch thing.
Open My Files app and browse the Internal Storage folders, you should see there all your phone's files. Look for any folder that may sound that have been created via the Smart Switch transfer you did when moved from iPhone.
It's clearly showing that your System/user data is using that amount of space but it makes no sense as mine, with over 100 apps installed and the OEM plus the rest of other usual files for me are using just 18.9 GB of storage.
I'm afraid that if by browsing your folders and files you don't find the obvious reason of this you will need to do a factory reset, I'd suggest you to at least use the Samsung Cloud backup so you can get back your Home screen and common settings plus any video, pictures and music you may have.
I heard from another thread doing the smart switch is a bad idea as it transfers all your data + corrupted files or any junk you had previously now i haven't done this with an iphone but i'd guess that could be one of the reasons why. I personally did the smart switch from my note 4 and resetted the phone couple days after besides my battery life which became a bit better imo didn't seem to affect anything else but then again i mostly start fresh with any phones :s
I did a smart-switch from my note 4 to my s8plus didn't encounter any space issue however i noticed that some apps didn't get installed so no big deal i just re-download it from playstore. I originally try to pulled all my apps from my account but found it taking forever where the smart switch was much faster.
I used the smartswitch and noticed that the SD card on my old phone had been copied to my new phone's internal storage as a backup. Once I got rid of that and some other fluff I now have less than 17gb used. I'd say you have some serious overhead somewhere.