Hi Adrian
There are 2 processes going on here
1 casting
2 Moulding
Casting a bust or a figure makes no difference at all,,,you pour resin into a mould and degas it. Its that simple. You need some practice and a few tricks but in general its a boring mundane task.
Moulding on the other hand is a totally different process and it is here you will see the differences. It is here that you are seeing the quality aspects and improvements of late esp from some manufacturers . Like Ian says figures can have more parts but half busts can too so its not down to that either.
Busts are bigger it makes them easier to mould I suppose,,,you can have bigger feeds into the parts and they are less likely to break or fail. Personnaly I don't find any difference big or small its how its done that matters. Good production comes from good moulding and good casting giving you what you want,,,a good high quality product be it a bust or a figure. It is why I suppose you are seeing the rise of casting services. When I started you had to learn to cast etc and we all did. There are very few now that do cast there own products which is a shame really.
Figures do take a bit more time to mould but this is rewarded later during casting giving less fails etc so yes they are more involved. Some of the hardest pieces I have moulded were busts !!!!
Moulding figures is generally a doddle, I like mine to be in a smaller parts count,,,why have an upper body, lower body , 2 boots etc for a straight up and down figure,,,it makes no sense unless its needed due to the uniform or the pose.
So difference for me ,,,no, each has positives and negatives which on the whole cancel each other out.
A bust or a figure,,,,,,,its a model,,,,,build it,,,paint it,,,,enjoy it.
Stuart