Some thoughts on chinese ballistic missiles. Various onlines sources often regurigate data that df-31A is longer than df-31. By some 4 meters. How did they get to that figure is beyond me. When one actually compares the two launch containers, using various available images online, it appears df-31A container is more or less the same dimensions. If it's longer, it's perhaps by half a meter, not more. Of course, range figures are impossible to deduct, so one either has to believe various sources like Jane's and their collaborators like Erikson, (CSIS missile threat uses their figures) or the figures mentioned by US DoD report to congress. The latter mentions 7200 km range for 31 and 11200 km range for 31A. CSIS mentions 8000 km for 31 and 11000 km for 31A.
Since the actual size is similar, the only differences imaginable are: last stage propulsion, as df-31 had a quite slim last stage. Materials/weight of whole missile, as 31A may have used lighter materials. And payload weight. If a more miniaturized warhead was developed, DF-31A might be using less payload, which would propel the warhead further with same engine. Or, theoretically, the engine itself could have been made bigger with a smaller payload.
Another way to estimate range is to use other missiles:
Bulava. 1-2 meters shorter, similar diameter. Allegedly can deliver multiple warheads to 8300 km, using CSIS data. Also reported tested at over 9000 km. Possibly/probably with smaller payloads. Bulava was developed from knowledge gained with Topol, (soviet first solid fuel icbm? if we categorize Pioneer as IRBM) Finally, after decades of low funding in 90s and 2000s it became operational from 2011 onward.
JL-2 was also tested in similar timeframe through 2000s and became operational in early 2010s. Now, it may or may not feature similar solid rocket fuel tech, or materials tech.
There is also US Trident I, as comparison. Tested in 1970s, deployed from 1979 onwards. It's a 1.8 m wide missile, 10.2 m long, so smaller than JL-2. It is credited with range of 7400 km.
Trident II, slightly larger than JL-2, (13.5 m long, 2.1 meters wide) deployed from 1990 is credited with range of "over 12000 km".
Basically, when one looks at hike in range claimed for DF-31A, without a big size increase, and when one compares the other missiles, which, save for Trident II, aren't necessarily meaningfully more advanced, It seems quite plausible that JL-2 range, with a single warhead (weighing a few hundred kg) is actually 8000+, possibly even reaching 9000 km.