Is it unusual for my testes to be very different in size and shape?

No. It is very common and normal for testicles to be slightly different in size. This does not increase chances of malignancy in the future providing the testes are anatomically normal (i.e. No masses, lumps, nodules, etc.).
Yes. If you have testicular asymmetry, then either one testis is larger than normal or one is smaller than normal. Size discrepancies can be normal however. Larger testes can be due to tumors; smaller testes due to prior trauma, infection, surgery, or a varicocele. See your urologist immediately.