The twenty fils coin is smaller than the 10 fils coin. I discovered this because of a toy ATM machine my son got from the Ahli bank of Kuwait. This machine has a little card and you can deposit coins and notes. A PIN number is required to make a withdrawal and the denomination of the notes need to be entered to calculate the balance. The coins however are counted on the basis of their diameter.
Thats when I realized that the 20 fils, which is recognized as a 10 fils coin is smaller in diameter than a 10 fils coin. So you need to deposit a 20 fils coin along with a 10 fils coin and vice-versa to ensure that the balance is correct. The 20 fils is obviously higher in value and made of a ‘shinier’ alloy than the 10 fils coin, but why the difference in size…I don’t know