In North America, the load rating on the OE 15" tires (tyres) is 89. The 17" tires are 87 but they have a higher speed rating which may compensate for the lower load rating. In general you want to stick with the OE load rating or higher. Speed rating is the letter following the load number. So T is a lower speed rating than H and H is lower than V and so on. I prefer the T rated tires when available but the difference between T and H is not large.
195/65R15 has a load index of 91. The standard 215/45R17 is 87. Fortunately the 16 also comes in 91 and for me going from 15's to 17's meant for me to maintain my roadworthy status, I went the 91 which was fine as I wanted a sports tyre. Prius is fuel efficient enough to live a little and at the same time increase the grip circle vs any Eco tyre.
87 load index is ample considering rather low weight capacity listed for the vehicle, i.e., 820-860 lbs.
