Surely, you've noticed PCS and NPCs in Gamma World, and Mutant Future have a lot of Hit Points? You must have also noticed high tech weapons and some mutant attacks do a ton of damage but old-fashioned archaic weapons do pathetically low amounts of damage? Here's how I deal with it.
Leave the HP as is in the rules. They are fine the mutations, radiation, and mutant power rules need PCs with that HP range.
I like a campaign where atomic gladiators, wasteland scavengers and teenage mutant cavemen deal with archaic and makeshift weapons throughout the life of the campaign. So here's how I make that work with the high HP nature of the game: rate weapons by overall quality and key damage to that. I do that by having archaic weapons grouped into Junk, Primitive, Advanced, and Powered categories.
Junk Weapons are cobbled together by relatively unskilled crafters from inappropriate materials, old gift-shop weapons and mistreated salvaged relics. You have a table knife strapped onto a shabby old broomstick well that a Junk Spear. Junk weapons will do the same damage as listed in the rulebook. Any character can make a junk weapon with relative ease.
Primitive Weapons are fashioned with skill from ancient materials, this can be stone or bone if the crafter is skilled in using those materials, or from simple alloys and salvaged metals if a crafter has access to the right type of workshop, tools, and appropriate skillset. A Steel spearhead mounted on a reinforced aged wooden shaft would be a Primitive weapon. Primitive weapons do twice the damage listed in the rules, if listed as 1d6 in the rules it will deal 2d6 damage as a primitive weapon. Trained characters from the appropriate tech level with access to tools and materials can make primitive weapons.
Advanced Weapons are fashioned with skill incorporating advanced materials beyond the capabilities of unorganized societies low-tech societies. Specialists with the right facilities, materials, and tools can manufacture Advanced Weapons. Advanced weapons deal three times the damage listed for archaic weapons in the rule book, if listed as 1d6 it will deal 3d6 damage as an advanced weapon.
Powered Weapons are fashioned incorporating mostly advanced materials with energized or mechanical systems requiring high tech capabilities and supplies. A Plastisteel Blade with a phase inducer and charged power cell would be a Powered Weapon. Powered weapons deal 4 times the damage archaic weapons do in the rulebook, if it is listed as 1d6 it will deal 4d6 damage as a Powered Weapon.
Notes: "but my rule book has separate damages for small-medium/Large creatures" Either do the change for each size class or just use the small-medium damage range and multiply that.
Junk Base Damage Dice x1 value
Primitive x2 Damage Dice x3 value
Advanced x3 Damage Dice x10 value
Powered x4 Damage Dice x50 value
Use the guidelines above and your scavengers and survivors will be getting more use out of archaic weapons in your campaign and there will be a wider range of loot available with only a little extra paperwork.