I've had the bag get stuck with larger grain bills. It helps to have a ratchet pulley. Hoist up as far as it will go, work the sides of the bag with mash paddles, hoist up a few more clicks, repeat. I did a batch with a 24 lb. grain bill. It only took about 5 minutes to get the bag out, which was no big deal, since it needed to drain anyway.
Make sure the cutout rim of the keggle has been sanded smooth with no burrs.
I just can't understand how you figure a reduction to 13 gallons, eyeballing the uncut kegs I have it looks to be less than a gallon lost, or 14.75 gallons post cut...hate to nit pic, but IMO every keggle should be fully opened IMO.
I have sold many, many keggle bags with positive feedback and have never heard of users having difficulty pulling the grain bag FWIW. Keggle bags $25 shipped.
I started with a keggle with a cut hole. Then reached underneath and cut out the lip. The cut more off to make it lighter, then sold it for enough money to buy an aluminum 15 gallon pot that's way lighter!... Lol. It was killing my back! Lol