If the output voltage is equal to the input voltage, then a turns ratio of 1:1 should be good enough. If you have feedback from secondary back to primary and a 1:1 turns ratio then the duty cycle should settle down at approximately 50%. If the turns ratio is higher (1:2) then the duty cycle will change to accommodate this.
There are 2 things to consider: if you have high turns on the primary and low turns on the secondary, the secondary side experiences a high current, but the primary FET experiences a low voltage stress. If the turns ratio is low on the primary and high on the secondary, the primary current is high, but the voltage stress on the secondary rectifier circuit is low.
Having a duty cycle equal to Vin:Vout, balances the current and voltage stresses.
Now, this is true for flybacks. It has been ages since I designed a forward converter, but I think the theory is true for this