Leverage.
Take a full size BH, a mid size BH, and a Super Single-Six all with factory springs and cock them.
The full sized BH will feel easier, the mid size BH a bit harder, and the Single-Six the hardest of all.
The reason is the height of the hammer spur. The shorter it is, the less leverage you have. So it feels harder to pull.
Same with the Bisley and SBH hammer, they sit lower so feel harder to cock.
No big deal, just use them and perhaps get a hand exerciser to increase the strength of your hand.
Joe