We likely have the physical capacity to make much, much bigger bombs, but part of the reason we moved away from just making bigger blasts is that they became impractically large long before we hit Tsar Bomba size. It's highly unlikely anyone is making anything bigger than Tsar Bomba, or even close to it. There's literally no conceivable case where a 50 megaton bomb is remotely necessary, at least shy of an Independence Day-like alien invasion lol. We can be just as effective with a fraction of the yield, and even a "tiny" nuclear bomb is both devastating beyond belief and basically a guaranteed an escalation to end-of-world event simply due to the quantity of bombs.
