It is only the 77 that was 25 mm, the later ones are 26 mm. The 80s were always 30 mm.

Because the 77 stroke gives enough swept volume in both versions to make far more power than 12 fpe, the 26 mm needs an even weaker spring than the 25 mm version. This means its lock-time is longer than the 25 mm, meaning it is lazier to fire and some people find it less easy to shoot accurately than the 26 mm. It is a subtle difference however, and there is not much in it unless you are a fanatic.

Basically the 25mm ones are more efficient and 'more sudden' to fire at sub-12 fpe energy levels. Some people believe the build quality of Weihrauchs in that era was higher than the modern offerings as well.