Er...the 50MT device was a cold war dick measuring stunt, not the demonstration of a practical strategic weapon. But, yeah, we (the US) don't even build 1MT devices any more, because 1) we have to fit them into submarines and bombers where size and weight (and economic) requirements limit yield, and 2) we figured out pretty quick that due to diminishing returns on destructive footprint, bigger bombs were pretty much a waste of effort (and produced other side effects like more fallout). Among other gotchas, you're constrained by the cube-root law; ten times more yield is not anywhere close to 10 times more destruction on the ground.
So, since we saw first-hand what devices of ballpark 20KT yield did to an urban environment, numerically most of the operational US nuclear weapons are around 100KT (margin of error and all that)[1]. So, yeah, better targeting, smaller warheads, yada yada. Still one-shot/one-kill for an entire city and the surrounding area. I mean, McNamara could handwave with statements like "optimistically, only millions dead" (paraphrased), but I don't think that kind of thinking has aged well.
That said, things are not worse because the Big Three have Big Bombs (or don't). It's because the strategic landscape has changed. One of the big 3 is lead by a guy who thinks nuclear weapons are just like big hand grenades or something and that negotiation is a zero-sum game. Another bit three leader is maybe just holding on to power by his teeth (or not...who knows) and came of age in the 'nuclear war is winnable' era. And there are more players at the table. Israel has nukes. North Korea. France and the UK. And India and Pakistan. South Africa had them before, and could again. And Iran will get them at some point, sooner or later. And we're apparently going to give the Saudis all the required tooling and expertise required, though there's apparently a firm pinky-swear no weapons will come of it. So we aren't looking at a 'simple' US/USSR cold war, with some Chinese spice in the mix. We have regional power, some of them not known for long-term stability, some with extremely volatile regional situations, with the ability to knock over the apple cart.
So what is the escalation/deescalation path if, say, KYI has a real bender one night and decides that NK nukes Seoul? Or Tokyo? Or what if they just pull a June 1950 and march all the way to Pusan conventionally? Do we nuke Pyongyang? All their nuclear facilities (the ones we know about)? Or glass over the whole place? And what then does China do, us having set off a nuke in their literal back yard on top of their Little Buddy? Under such circumstances, I do not expect the current administration to take a measured step back and say "we need to think this through...".
Uncertainty. That's why a lot of us (who lived in the Cold War) think things look a little shakier than usual.
[1] To head off the inevitable "well ackshually...": Yes, the big players still have big bombs. I haven't been involved in this area in a while so things have probably changed a bit, but we still have some of the big boys in the 300KT+ range sitting on Minutemans for old times sake. I assume the Russians still have a bunch of their 1/2 to 1MT devices in silos. And the Chinese were pretty proud of their absurdly large 4MT+ devices, supposedly still in service.