There are so many variables to consider when you want to acquire a smooth texture on a papier mache surface. First of all, what kind of glue are you using? It can make a lotta difference in the amount of shrinkage you get. it took me awhile to figure it out many years ago, experinmenting with various options available......off hand, I'd say that it sounds as though your glue is way too wet and shrinks when drying.
Also.....remember that if you were to ask the 100 best papier mache artists in the world for their own "tips & hints" on a process or type of material........they would probably EACH have their own unique formula and might even tell you that THEIR WAY is the RIGHT way.
The point is, that while you can benefit from other artists' experiences, the bottom line is that you have to be willing to try a lot of different things until you achieve something that works for you. I use a mixture of wallpaper paste with polyvinal acetate (white glue) and have minimal shrinkage. I have numerous books that recommend other types mixtures and types of adhesives.......
As for surface smoothness.......I've heard of artists using plaster, wood putty, gesso, texture paste, wall texturing materials .......a good place to check out possibilities is a good hardware or paint store. Nowadays, there are all kinds of possibilities designed to add textures to walls.......these things also work on papier mache surfaces. Sanding helps, too. It is possible to make a papier mached surface look like ANYTHING!!! I have figurative papier mache sculptures that people always think are bronzes!! Or stone.......believe me, it is one of the wonders of working with paper!
I have to admit that having 40 years sculpture experience, including over 30 years working with theatre and art of illusion has provided me with the tools of the trade........most of the materials I use for working with papier mache come from hardware, building supply and paint stores. I call them my art supply stores.
Good luck with your mask making!