<p>If I switch on the generating of debug info with Javac then the class files are 20-25% larger. Has this any performance effects on running the Java program? If yes on which conditions and how many. I expect a little impact on loading the classes because the files are larger but this should be minimal.</p>

In any language, debugging information is meta information. It by its nature increases the size of the object files, thus increasing load time. During execution outside a debugger, this information is actually completely ignored. As outlined (although not clearly) in the
JVM spec
the debug information is stored outside the bytecode stream. This means that at execution time there is no difference in the class file. If you want to be sure though, try it out :-).
Ps. Often for debugging there is value in turning off optimization. That
does
have a performance impact.