When I had my corset classes, the big students were always worried that they wouldn't be able to sew a corset that fitted or that they wouldn't be able to get into the corsets they actually had sewn. But the thing was that it appeared as the more generous the figure, the better result when the strings were tightened. The women had suddenly gotten a figure with a very noticable waist and a breast lift as well. Of course, women with a slim figure also got good results but the result was much more obvious for the bigger figures. So, I say, it is more satisfying to sew a corset for a fullgrown women. However it requires more to finish a corset as there may be needed two or sometimes even three mockups before getting everything right.
Yup, you're my patronus, Rosie -- THANK YOU for writing this!!!! I'm currently working on a corset for a plus sized client who's quite a bit larger than me (and I'm a North American size 18). It's been a challenge to ensure that the pattern fits wonderfully over her torso flesh, as there are definitely dips in areas that there aren't dips on tiny sample sized bodies! But I'm so excited by the progress we've made, and the client knew from the get go that I hadn't made a corset for someone her size before, and has been great for accommodating the multiple mock-ups.
I'd like to add to your list, if I may -- make sure to have a tape measure that measures more than the 1.5m that most do. I think that you can get quilting ones that are 2-3 metres long. I was SO mortified at myself when my tape measure I'd brought didn't fit around the client's hips! It's the same effect as you mention, with trying on a mock-up that's too small!