Not that I question the number of nuclear weapons Pakistan might create over the next decade, to suggest they will exceed China is truly questionable since many analyst question how many China have now (as in that the official number is too low). And further to believe it would stay static as Pakistan makes more nukes is in itself questionable (I'll throw in China just tested a new ICBM with Multiple warhead capability). But the bigger question I have is why this article would list Uranium weapons as low yield and Plutonium as high yield. From my research its the other way around.
When you use Plutonium you have a maximum amount of material you can use of around 18 lbs. Some would say I am wrong since you need 22 lbs for critical mass, but thats a ceiling you must never reach. When using an implosion design all the material is in one big ball shape at the start. Thus if you use a critical-mass of material, it will chain react in our face as you finish assembling the bomb. And if you try to use all plutonium in a gun assembly the chances are very high you will get a fissile (a very low yield). So the max yield of an all plutonium weapon is around 40 kilotons.
For uranium the opposite is true since the critical mass of uranium is 128 lbs. . This means you have more material to fission allowing for a higher yield. In the late 40s, a shortage of Plutonium from Hanford caused the US government to experiment with mixing Plutonium and uranium into one core. The result was we could produce more bombs in a given time frame and the yield of the bombs went up compared to their pure plutonium versions. Then in 1952 we completely replaced the Plutonium core of the Mk-6 bomb with uranium (as much as we dared pack in). The result was the Mk-18 or Super-ORalloy with a yield of 500 kilotons. This device was tested in 1952, shot King, Operation Ivy.
Thus my concern over the source of the information for the Washington Post article. Like the Daily beast report, which suggested we had enough material for over 12 bombs, when in truth we barely had enough material for three additional bombs (and that was by the end of the year). Nuclear myths seem to be the norm now given the continuing myth that suitcase nukes are impossible. Its what truly makes me worried about this Iran agreement, just how much of this agreement is based on modern nuclear mythos.