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I've been contemplating moving my BOB to a book bag for this very reason...unfortunately it's full of pockets and notebook sleeves and water bottle holders so fitting in the big stuff I'd want to bug out with isn't that practical.
I've found that I could pack much more stuff in such a bag if I organize items in tall flat packs to better utilize the top side of the bag. Also they are stretching quite well, so a hard box placed in the external pocket or inner compartment could help to pack more.
I tried doing this last night with the BOB I'm formulating in another thread...it's full (tight full) without the sleeping bag even though it's about the same size on the outside as my German military rucksack. The difference is the ruck sack is just one big empty pocket. The other issue is that compression bags, stuff sacks, etc, don't go flat...and because everything is in a kit, the little pockets and flaps are dead weight.
The best pack for the job so far believe it or not is my Mountain Hardwear scrambler even though it has the smallest internal volume....I'm learning that shape is every bit as important as volume. I wish there were compression bags which compressed stuff flat like a book for the specific purpose of jamming soft goods into book bags.