My guess is a lot of people can't afford several backpacks; they use their main backpack for most of their needs.
I use an Osprey Switch 40+5 backpack, which has been designed for snowplay. I use it for snowshoeing mostly and when I get back from my trips, the pack gets emptied completely.
When I get called for a SAR deployment, I simply transfer my SAR items from a small box into my backpack. My goal being that I want to know exactly what I have available when I put my pack on my shoulders, wether it's a SAR operation or a snowshoeing trip.
So there is no limit to the size of your kit, really. It depends on how you wish to work with your gear. I am comfortable having an empty pack, with nearby items to fill it up. I have deliberately chosen not to have a full pack ready 100% for a bug-out.
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"The only easy day was yesterday."