According to this thread, Fat Man and Little Boy - the nuclear bombs dropped on Japan in WW2 - weighed around 10,000 pounds each, and had yields of 15-21 kilotons.
In contrast, modern nuclear weapons have a yield many times larger, and yet we are able to cram a dozen of them on top a single ballistic missile. The claimed weight for a W88 warhead (with a yield of up to 475 kilotons) is 800 pounds, so they each weigh about 1/12 what Fat Man/Little Boy weighed.
Since a W88 is a fission device combined with a fusion device, that means the fission device is much, much smaller than Fat Man/Little Boy. How is this possible? Don’t you fundamentally need X amount of fissile material to achieve critical mass, and Y amount of high explosive to give it a hug?
Think of the difference between… a blunderbuss and a caseless H&K SMG. Same basic device, same basic result, one hell of a lot of engineering, chemical and other refinement in the latter to do a lot more with a lot less.
Or, you can get a fission pop out of a very small mass if you implode it with more force and precision than Trinity did.
I don’t know much about nuclear weapons, but suggest the following:
you are comparing the weight of a bomb with its payload to just the payload itself, but this is probably not a big part of the difference.
My understanding/memory of history is we didn’t have a whole lot of uranium around which we could test designs with. We had to go with what we were sure what would work - so probably the simplest design - which wasn’t most likely the most compact. Once we had extra uranium/plutonium laying around - we could afford the testing needed to make these smaller.
We’ve made plenty of advances is materials and electronics. For example - depending on how many vaccum tubes (if any were needed) - they now could be replaced with lighter weight transistors (maybe not in all cases). Also we have made plenty of advances in lighter weight composite materials for structural components.
Just my guesses and I’d put my money on number 2 being the most important.
It all comes down to engineering.
Better fissile material (higher purity, less contamination with PU-240), allowing more compression and hence higher efficiencies. Better explosive lensing, and detonator timing, allowing more core compression, tritium gas boosting - resulting in more efficient fission, etc, etc.
The smallest American nuclear weapon put into production was the W54: sub-kiloton yield. Weight: 50 pounds. 11" diameter core.
Before 1962. Both the Army SADM (“backpack nuke”) and the Air Force AIM-26 Nuclear Falcon were in active service at that time, and they both used that warhead.
The basic design was being reliably verified in the 1961 Nougat test campaigns. That’s 16 years after Trinity.