You'll quickly find that the space is enough as you progress. Yes, when you're in the first days, it is limited. However, as you progress, you'll get better and better vehicles, which each increase space from the previous vehicle (except gyro). And although those don't go into the POI with you, they can still be placed within the POI radius so you can fill them up in the middle of a quest if you really need to. You also can eventually buy or craft a drone and then, based on the drone's level, add up to 4 cargo mods to increase storage even more. And that will follow you around a POI. By the time you're into the game a ways, you'll have plenty of "mobile" inventory space - your inventory, drone inventory, vehicle inventory - to be able to get what you want and then unload back at your base. Now, you might not be able to scrap an entire tier 5 without running out of space, but if you're just looting, you should have no trouble looting even 2-3 tier 5 POI with that amount of space before having to either choose what to keep or going back to your base to unload. At least as long as you're stacking everything well and don't have only a partial stack of something in your drone and your vehicle and your inventory, when it could all fit into one stack.
With this in mind, it would not be good to increase backpack space even further because you already have a lot of space available through progression. If you really want more, and are playing on PC, there are backpack mods that increase space. But it really isn't a bad thing to have to choose what to keep or what to either throw away or come back to get later. Besides, stuff you leave doesn't go away (unless it's in a dropped bag or pouch), so you can always return to pick it up later if you need to... as long as you don't reset the POI with a new quest, of course.
Now, I have started using a backpack mod due to using the Vehicle Madness mod that adds a lot of extra loot and so I need extra space to handle that. But for vanilla, the available storage works well.
