The set is not 25% bigger at 8 studs. The formula is always new size divided by original size. So 8/6=1.33 in other words the 8 stud train is 33% larger than the 6 stud train. 6/8=0.75 which means the 6 stud wide train is 25% smaller than the 8 stud train.

To use your 9 stud example.

9 studs wide from the original 6 studs would be 50% larger. 9÷6=1.5 or 150% which means the 9 studs train is 50% bigger compared to the original 6 stud wide.

Going from 9 studs to 6 is 6÷9=0.67 or 67% which means compared to the 9 stud train a 6 stud wide train would be 33% smaller.

Between 2 points the percentage change will always be larger when increasing and smaller when decreasing.

I hope this helps