All nails are different, and I don't think that there is really a way you can make them thin. So the best thing to do is to simply find a way to treat them them right.
The best you can do, is to get toenail clipers, so you have something that wont break and stuff, when you cut them make sure that you soak them in warm water so they don't seem like such a bother, and just keep them short.
The thickness of toenails tends to increase with age (particularly as you enter "middle-age"), and there is no really effective, practical, way to reduce the thickness.
It is best to use good nail clippers to keep the nails trimmed to a convenient length.
Best wishes - Majikthise.
Sudden thickening of nails may be due to the onset of nail fungus. You should see your doctor at the first signs of nail fungus to begin treatment as soon as possible-the sooner it is treater, the better chance it will clear up quickly.