1. All things equal will a 120G SSD with 30G of data on it have 2x or 3x the number of writes than a 60G SSD with 30G of data on it?
2. Does wear leveling mean that even untouched files are moved around?
3. If an SSD uses its extra space for wear leveling, why does it help to convert some of that space into extra wear leveling provisioning space?
1. Yes. (the exact number is up for debate but it's more than 2x)
2. Yes, it's called static wear leveling (otherwise you would end up with uneven wear on the drive and lower lifetime).
3. I'm just speculating here but I think it has to do with the scheduling of the cleanup; having a certain number of free blocks (some only temporarily) is not the same as being able to count on having them always free. Also, if TRIM command is not supported, once you write to a block it remains a used block even if you delete the data afterwards, so by limiting the usable size you make sure you never write to a certain number of blocks.