They'll allow some growth, but not much.
If you pull the mandrel out of the die, you'll notice a step near the top. The edge of that step contacts the case mouth and pushes it back slightly when you use the die on the case a second time. This minimizes growth. They'll stretch a few thousandths each time you fire them, but by the time they're back to trim length, the cases are probably used up anyway.
I've been using the small-base X-Dies for .223, and they work as advertised, but the big problem is that you need to keep your X-Die-sized brass separate from any range brass you might pick up. If not, and you use the dies (as instructed) for a second sizing, than mandrel will squash and ruin any case that hasn't been trimmed to 0.010" under trim-to length.