I would probably not use CDC data for this. From what I found for Japanese women:
Among the women with height ranging 149 cm and less, 79% of the women have their waists in the range 56 cm to 58 cm and the weight is in the 35 to 39 kg range. In the 150 to 159 cm height group, the weight range is 40 to 45 kg and the average waist size is 61 cm.
And saber is listed as 154 cm, so her weight would be perfectly average for an adult woman.
It is a Japanese game. I would say that almost none of the characters actually look they are from the place they should be. If her story makes you uncomfortable, that is fine. But I wouldn't base it solely on one throwaway piece of information that you probably couldn't even find in the game itself.
Canonically, she is supposed to be probably 25. And I would say that is a reasonable portrayal of someone who was forced into being queen at the age of 15. Again, completely fine if that makes you uncomfortable, but if you are uncomfortable with this, you might have some more issues later in the game. (Not sure how far you have gotten.)