iMac storage issue/huge recovery partition

Asked this question at the iMac thread, they suggested people here might help out as well.. So here is another thread on the same issue:

So, I bought a used iMac. The guy had Windows installed on it. I did a clean install of High Sierra, erased the disk and everything, it worked nicely. Until I discovered that my 1TB disk shows only 200GB available storage.

I discovered there were 3 partitions to the disk, and after deleting one, my main disk had cca 600GB of space. But after deleting the second, my disk remained at 600GB and the other partition dissappeared. Then I decided to do another clean install, but after erasing the disk I couldn't install the OS anymore, so I took it to get it repaired, and they installed the High Sierra again.

It's working nicely and all, but my storage is still at 600GB. When I go to "partition disk", however, it shows 1TB of disk size. (pics included)

I ran the diskutil list in terminal, showing that I actually have a Recovery Partition that is 362 GB in size!! ??

I am guessing this is the third partition that couldn't be deleted through disk utility and used to have windows on it...

So, now when I run Bootcamp, there is nothing to do, I cannot remove the windows partition (which doesn't exist anymore, but has turned into a huge recovery partition)...

I know I made a mess, but please help.