http://fallout.wikia.com/wiki/Plasma_rifle
As you can see, it remained consistent in Fallout 1, 2 and Tactics. (Fallout:Brotherhood of Steel never ever existed and lets not talk about it), and then suddenly in Fallout 3, it's design radically changed. It was justified in cannon as the weapons on the east coast are from a different military supplier than on the West Coast, but then this is completely thrown out the window with New Vegas, a small portion of which is actually on the same area of land as in the First games, with armor and equipment previously used and abused by groups such as The Unity, NCR, Enclave, and Brotherhood. If the Midwest has the same weapon's supplier as California, why the hell would Vegas have guns from Washington and nothing from the West Coast? I personally think this is just laziness on their part as newly created models look the same as the originals yet the reused weapons are just copied from the Fallout 3 folder to the New Vegas folder. And for crying out loud, the old plasma rifle just looked amazing.
Weapons like the assault rifle can be justified because the looks were simply taken from 2D to 3D and given additional details, aside from the 10mm which is again am Eastern brand. Also, still no Desert Eagle? Shame on Obsidian, though at least New Vegas unscoped the .44 magnum, which I will say was my longest lasting weapon in F2.
Now onto armor, this isn't so much of a "brand" issue, but merely one of general appearance and how it's used in the newer games.
We all have our separate opinions on whether or not this armor is horrifically ugly, kinda meh, or the most beautiful thing to come to the wasteland, but the Enclave armor in 3 still majorly irks me. Now you can excuse it as "Well they salvaged T-45d and painted it black". This might make sense if you want to justify its stats which are much lower than advanced power armor should be, but ...why do they look like steam-powered batman? Where did those helmets come from? Why does the body look "rule of cool-ified", why is it so clunky, and where did the Enclave leave all their older, better suits?(apparently in New Vegas). Now, I'm of the opinion that NV should just completely retcon Fallout 3's Enclave armor and say it never happened, because whatever you think of it's looks, it came out of nowhere with no basis in lore. If you read quests or just listened to Sulik, the specific purpose of the appearance of Enclave armor is to look like "Bugs that walk on two feet" to "[Scare] the locals" away from their mysterious plans. Well, I guess the Enclave in 3 are joke anyway, so you can't take them as seriously as before.
Okay, enough about that specific piece of armor. Now about the general "bulkiness" of power armor. If you ever played the first 2 games(Tactics is an exception, they made the same mistake), you'd notice that the sprite for power armor is significantly larger than the other sprites for characters You don't wear power armor, you climb inside of it. It's not a superman outfit, it's an exoskeleton, a man sized tank and it is by no means "plate armor". Plate armor is worn, and has a mail underlay to protect the joints which are otherwise unarmored. Power armor, as it's name implies, is powered by electricity to move around, the user does not simply walk into Mordor it like a coat of plates. If you take a look at even the bulkiest armor in the game, advanced power armor/Remnant armor it seems to fit snugly to the wearers body, getting thinner at the waist, which is not what it looked like in the other games.
Most of these issues I'm okay with being in Fallout 3, but I'm miffed that the mistakes are being continued into New Vegas and likely future games.