Ah, sure! It’s all about lever arms. Torque = Force x Distance.
That drag on the supply reel is a torque. To overcome it, you need to apply some force at a distance.
Since your torque is constant (the drag on the spindle), only your force and distance can change. And they are “inversely proportional” meaning if your force goes up the distance must go down. And vice versa.
So, when the reel is full and the film is near the outside diameter, that’s a big distance. So your force (tension on the film being pulled) will be low.
As the film gets to the end and it’s near the center of the hub, that’s a small distance and your force (tension) will get bigger.
You can also think of it like a wrench. It doesn’t take much force to turn a bolt with a really long wrench. But if the wrench is short, then you’ve got to put a lot more oomph into it.
That translates to there being low film tension at the beginning of the film (large diameter) and higher tension at the end of the film (smaller diameter).
In a perfect world, you’d want some clever linkage that could alter the drag and keep the force (tension) constant.
In an 80% perfect world, you may just want to set the drag at one acceptable level over the whole run of the film (knowing that the tension would change over the run, but that the film could handle it and it would be acceptable).