Platinum rings can be resized, although doing so costs more than resizing a ring that is made of yellow or white gold. The reason that platinum rings cost more to resize is because platinum itself costs more and the process is more labor-intensive.
Platinum is more difficult to resize than gold because it has a higher melting point, and reworking a platinum piece requires many special tools. Tools that are used to alter platinum must be used exclusively for platinum jewelry or the platinum will discolor. Resizing platinum rings also requires significantly more skill than resizing gold rings. Most jewelers provide a price quote for the job before performing the work.