By considering the grid lines in the image, you would see that 1 box represents 1 unit.
1 box = 1 unit
For card 5:
The small circle spans 2 boxes
Size of small circle (A) = 2 units
The big circle spans 6 boxes
Size of big circle (B) = 6 units
The Scale Factor, SF (A-B) = 2 / 6 = 1 / 3
Therefore, SF (A-B) = 1 : 3
For card 6:
Size of A = 9
Size of B = 3
SF (A-B) = 9 / 3 = 3 / 1
SF (A-B) = 3 : 1
For card 7:
Size of A = 4
Size of B = 2
SF (A-B) = 4 / 2 = 2 / 1
SF (A-B) = 2 :1
For card 8:
Size of A = 5
Size of B = 5
SF (A-B) = 5 / 5 = 1 : 1
SF (A-B) = 1