My experience with larger packs is I can't resist the urge to fill them.
I've had some truly enormous packs over the years (e.g. Tactical Tailor's MALICE V3). At one point, my pack weighed in at 67 pounds. While I could probably make this work, in isolation, it doesn't work when I add a rifle and ammo LBE. Now I try to keep my pack to around 40 pounds. My current set up is 43.8 pounds in a Kelty AMRON Map3500. This is about as high as I can comfortably carry (if I'm carrying an AR with ammo).
Having said all of that ... my pack selection was based upon my objectives.
If your area or plan dictates that you need to survive for a long period of time in an inhospitable environment you may need a larger pack. Before purchasing a pack I would ....
1) Write out my plan of action;
2) Write out the BARE ESSENTIAL gear I need to suceed at my plan of action;
3) Review and revise;
4) Buy my gear to see who big it is;
5) Delete or replace anything that seems too big / heavy / whatever;
6) Buy a pack to fit the above.