After having used the smaller 3.5" wide model for some time, I think (and can see) for the 4" model could be a better fit overall. Being longer and taller, it looks to fill out the main compartment better. That probably helps prevent some of the rear bulging since the whole back would be flat (assuming you aren't still trying to use the CP's rear pockets.
In the CP with the smaller Ape Case, much like you can see in my Pilot video, I have room to the side of the insert. I find that useful for storing other things while I'm out for the day, but I think it also causes some "fall off" in how the back of the bag sits. That side can cave in, creating a different kind of bulge if you don't pack right.
Live and learn! Maybe I'll buy a larger size to fit the Pilot better...
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