My 2 cents worth,

When the film holders get bigger, the dark slides get thicker, hence the area at the top in which light can enter the back is bigger. You might check the springs on your camera back to make sure they pinch the top of the holder (compressing the opening) when the dark slide is pulled. You might just need to apply more tension to the camera back.
If that doesn't work, use a couple of work arounds: 1. In bright sun light, leave the end of the dark slide in the top of the holder (put a mark on the outside of the slide where it is out far enough not to cover the film, but still fills the opening), 2. cover the top of the holder with the dark cloth and bunch it up against the darkslide, such that when the slideis pulled, the dark cloth covers the opening, reverse the process for inserting the slide.