No you really can't. Being able to factor 15 but not 21 with Shor's algorithm is normal. I know it sounds absurd, but it really is that way. Because factoring 21 is about 100x times harder than factoring 15.
My point was that the comparison with nuclear explosions is wonky, since we (in the world of that analogy) already have seen a tiny nuclear explosion 15 years ago. And we kept being told that explosions 100 times larger are just around the corner, but explosions 25% larger are way too hard to expect.
I get that there's a lot of R&D going on to make larger quantum computers a thing and that there's been very definite progress, but factoring 21 is just too hard to expect for now. But that also pushes the date where pre-quantum cryptography is broken further into the future. If we still struggle to factor one of the smaller 5 bit numbers, factoring the 128 bit numbers necessary to break elliptic curve cryptography seems quite far away.
See https://algassert.com/post/2500 for details.