I'm not SIA.
it is my understnading, SIA will be a storage provider on the file/object level, meaning that any computation will have to take place locally after retreival of the relevant data from the SIA net work. It is specefically mentioned SIA cannot do computational operations on the data - no one except the renter knows what the data is, and files are only brought back to their original physical shape after reteival in the renter.
@tjalemao I'm not familiar with mac but you need to delete the wallet file to start over. Another forum post says that ~/Library/Application Support/Sia-UI/sia/wallet is the location of the wallet file. Hope this helps. Alternatively if you only need the wallet function you can try my unofficial web wallet at the bottom of my signature.
The Sia-UI folder contains more than just the blockchain. For me it is 7.4 GB because I am also uploading a lot and my wallet file is large (old wallet with thousands of addresses). On a fresh install this is going to be way smaller. The 7 GB on your PC sounds about right.