Actually, some of the FNV armor weights are not quite that far off from plausible or realistic, if one considers how much armor materials and tools have improved after the middle ages.. Post nuclear FNV armors can utilize advanced metal alloy plates scavenged from multitude of sources including aircraft hulls, cars, robots and so forth; It is possible to both precisely craft (or salvage) thinner plates than medieval plate armors had, and also it is possible to get fairly decent protection value out of these thinner plates.
For example, lets take a closer look at the basic Metal Armor; It seems to have an underlayer of a black leather outfit, over which partial coverage steel plate armor is assembled, metal plating covers perhaps something like only two thirds of the wearers surface and leaves the joints free to move. If we compare this to about 60Lbs weight for a medieval plate armor complete with arming harness and all, which protects almost all of the wearers surface, then a Metal Armor weight of 30Lbs without a helmet is actually quite believable. (Whole another believability issue is that -1 to Agility for 'armor weight' of only 30Lbs - Joints are free to move, and Heavy armor category already slows down the running speed a bit in the game, so there really is no plausible reason or need for the -1 Agility penalty.)
Reinforced Metal Armor offers more coverage (still has no metal over joints), but uses more advanced materials and presumably thinner plates as well, it weighs the same 30Lbs but costs over four times as much as the basic Metal Armor outfit - This implies both better materials as well as superb quality workmanship - Indeed, a set of reinforced metal armor is much rarer find in the Wasteland than basic metal armor.
FNV Leather Armor might seem a bit heavy at 15Lbs, but it also seems to include miscellaneous additional pieces of metal like pauldrons and maybe other stuff as well - These are propably partially responsible for both the added weight as well as bumping up the DT to the relatively high value of 6 when frex compared to DT:1 of Merc Troublemaker outfit (Also leather, 8Lbs and DT:1 only). Reinforced Leather Armor (DT:10) propably gets to more DT at the same weight of 15Lbs propably by using lesser thickness of leather and more added small thin steel plates, some of which may be actually concealed inside the leather. No wonder it costs almost eight times as much as regular leather armor.
Vault Security Armors (101, 34 and any similar outfits like Sierra Madre armors) are fairly obviously simply bullet resistant vests (made of kevlar or more advanced futuristic lightweight ballistic fibers, with or without metal/ceramic inserts) worn over BDUs, with combat boots. 15Lbs of weight for most of these is also quite reasonable, when you consider that the whole outfit includes more than just the kevlar vest (boots, BDUs, elbow and kneeguards, trauma plates etc). Variance in DT is mostly due different ballistic fiber materials or better armor inserts - In the case of Reinforced Sierra Madre armor, it is possible that the BDU garments are also made out of proper ballistic fabric and all of the materials are more advanced (it is the most advanced light armor outfit in the whole game, after all). However, what does not make much sense is the very low cost of these pretty good armor outfits, only 70 caps for Vault 34 SA(DT:16) or V101SA (DT:8), only 400 caps for Sierra Madre SA (DT:16) and so forth. If anything, I'd expect these fairly rare pre-war armors to cost notably more than the Reinforced Leather Armor (more than 1200caps in full condition).
Now, I wont touch even with a long stick how plausible or 'realistic' the power armors in Fallout are; Not much larger than a man, surprisingly light, yet they are powered by an onboard nuclear powerplant for up to a century of use (with the necessary radiation shielding, such a powersource alone would be quite heavy, like a brick or a few made out of lead), that is simply put, pure scifi. Also of note is that the T-51b started out with 85Lbs of weight in Fallout 1 (still quite light weight for a powerarmor, I'd say), but in Fallout 2 and all games after that the weight of a T-51b was cut in half all of a sudden..