“Book bag” as in a backpack? Just making sure I understand the question.
To me a saddle hunting pack has to hold my saddle platform, sticks (however many you use), plus whatever gear and clothes need to be carried in, and do it without clanking around making excessive noise or snagging on brush, etc. To me thats bare-minimum.
If you are in a cold climate and hike in mile(s) at times, the pack may need to be 30+ liters to fit enough clothing so you dont sweat on the way in. Warm climate, a smaller pack will work. If you drag or can cart your deer, then no need for a meat-hauling pack. I also like to carry out deer much of the time to avoid a long/uphill drag or to deal with crossing swamps, so I like a smaller meat-hauling pack, and I carry platform and sticks in the load shelf.