I feel like it's been discussed else where but too tired to do a search.
I used to do a fair amount of wilderness medicine (ski patrol, search and rescue, and 911 ambulance responding to wilderness calls) My philosophy was that I had 3 different "kinds" of bags. The first was the basic medical gear that came with me whenever I operated in my day pack. Gauze, maybe an oral airway, triangle bandages, gloves, CPR mask.
The next level was a dedicated basic medical bag. I had one for ski patrol when I was working, I had another that my SAR team used. It had things like SAM splints, trauma shears, more airway supplies, trauma dressings.
The final was the mondo ALS bag. This was used for confirmed patient locations who needed ALS, with an easier carry in or the ability to put the bag in a litter with a wheel. This bag had IVs, meds, OB kit, O2 tank, BVM, everything. We had one on SAR, we had for on ski patrol for the medics to bring to a call. It was a full sized, probably 40lb pack.
Note that the gear that you carry tends to expand to fit the size you give it. So you buy a huge ALS pack you are going to tend to bring a bunch of junk you don't need.
So the first step in buying a pack is figuring out exactly how you are going to be using it. Is it going to live on the truck and be grabbed to carry in for just medical calls? Are you going to carry it all the time on the fire line? Do you have to ski/climb/run with it on your back?