the starter motor positive cable is thicker than the other postive cable and negative to earth cable that goes to the battery terminals.

is it possible to change the cable to a slimmer one as i cant find any battery terminals that will support a cable that thick.

or is it not possible due to a higher amperage that needs to travel through that cable.

hope this makes sense.

You should be able to buy a new clamp as these are fitted on larger diesels (HGV's Coaches etc) if you fit a smaller diameter cable this will restrict the amperage/power feeding your starter and could cause the cable to overheat.