why after varnishing, the decal shrink ?

In order to answer your question, it would help to know:

1. What is the subject decaled-make and type of model.
2. Did the decals come with the kit?
3. What did you use for varnish?

Due to their alcohol base, some clear acrylic coats can literally melt a nice decal if applied too heavily.

  Tom T

Yes, your Tamiya TS-13 is a spray lacquer, which has a very strong solvent that can even remove fingernail polish!

If you go to this link to the decalling forum, you can get a lot of helpful info:

http://www.finescale.com/FSM/CS/forums/641519/ShowPost.aspx

Many, if not most of the fellas here use a mixture of Future floor polish and other stuff. If you want more info about using that, you can just enter "Future" into the search block below,and start reading the references to mixing the floor polish product for models.

I definately would not recommend "going there again" in using a Tamiya spray lacquer over any decals, as even as I mentioned before a havey coat of a high alcohol ratio clear acrylic will literaly melt them also.

As for your decals, I would say they are "goners" and seek to remove them using lacquer thinner and a bit of scraping with a dull knife, sand the surface, touch up the paint, and seek to use some others.

Good luck

  Tom T