The only time I seen drywall mud shrink like that is when it is put on too thick. It shrinks as a rule, and when it is that thick, the shrinkage has to show somewhere. If it is set in thin layers the shrinkage occurs in its thickness. When it is applied too thick, say maybe 1/4" or thicker, then the shrinkage occurs in both thickness and in its width.
You can add more in thin layers that will fill the cracking, but it will shrink into those cracks too, so more will be needed to get them filled in enough to level everything out.
When adding drywall mud, if the surface is essentially filled flat with the adjacent surfaces, resist the tendency to add more mud than needed to fill the cracks. A few tight skim coats may do just what you need. Then sand smooth.