Don't worry about it. Pitch only part of the cake, so that the yeast go through their reproductive cycle, and you will have healthy new yeast to work on the new beer. If you use the whole cake, you will not get much reproduction, and you will have stressed yeast working on the new beer.
You should not continually re-use yeast from high gravity beers because you are more likely to get mutations with subsequent generations. Re-using once should not be a problem, but if done a couple of times, you could start to find the yeast has a different characteristic (taste, flocculation, attenuation, etc) from the original strain.