Your instinct to question that isn't wrong since weapons do tend to get bigger over time, and modern nukes are still a couple orders of magnitude larger than the ones dropped on Hiroshima and Nagasaki. But in this case nukes have actually gotten smaller than at their peak in the 60's or so.

It's not that we couldn't build bigger ones if we wanted to, just that there's really no point. Cold War nukes needed to be bigger because the delivery systems (early ICBMs) were only accurate to within a few kilometers. If your bomb might miss a hardened target by 5 kilometers, then it needs to be big enough to utterly destroy everything in a 5 kilometer radius. That means a yield of several megatons, maybe even bigger. The biggest one ever detonated had a yield of 50 megatons, and even that was deliberately scaled back from its theoretical potential of 100 megatons.

But modern missiles are much more accurate - probably within 100 meters or less. Even a "modest" yield of 1 megaton is more than enough to take out anything at that radius. And smaller bombs are cheaper, more efficient (with big yields a lot of the destructive power is wasted by going up into the atmosphere or even space instead of out), and easier to handle and deliver.