Okay so with Adenia's it really has a lot to do with how it was propagated and pruning early on, and the type of soil it was grown it.
Unfortunately a lot of Adenia you buy are from cuttings which will never truly form the caudex you are looking for. The reason for this is that plants are either male or female, which is less convenient to produce seeds, than for businesses to just take cuttings at a faster time and larger size intially. The swollen caudex comes from seed grown plant, and pruning early on in its life.
But you can develop some of that to an extent through pruning. I have done with with adenia and adenium cuttings. You can basically prune it very hard back to about a few inches above soil line. And It should produce denser branching from around that cut and promote the stem to increase in size.
But before I would recommend doing that, is I would make sure it is in a very fast draining soil mix first with high percentages of large particle size aggregates. This is the first step.
The reason for that being the first step is after the hard prune is made it will need to be on a higher fertilizer watering schedule. I do this with all my caudiciforms when they are young where they get put in almost extremely (80-90%) or instant draining mix (100% stone) and then get a 25-50% dilute fertilizer mix watering every time I water. The fast draining mix prevents build up of excess fertilizer in the soil while still being able to push faster growth development and better root development, and little to no risk of root rot during the dormant months in "juvenile" ages, which is a killer for young caudiciforms and pachycauls.
And with bajas comment about root pruning this is not super necessary and unless you actually know what to do and how to deal with it I would advise against it. That being said I prune almost all my caudiciforms especially my adenium and when I had a bunch of seed adenias. The only reason to do this is to develop radial roots and nebari. But this can be dangerous but with my adeniums I currently have at one point I completely cut off every root, I made a horizontal cut across where I wanted the base of the plant to be and where I wanted roots to form. This is also a step for bonsai pot training. But don't do this it's taxing on the plant and is a potentially dangerous process.
And don't prune the roots at time of repot just put it in a good mix.
