Fritzing was originally developed for instructing beginners. A number of us are stretching it in ways that weren’t originally intended (such as very small SMD parts). Sometimes that causes issues. As Old_Grey noted, autorouting even in the professional tools doesn’t work very well for anything except very simple boards. The trace size stuff is one of the things I’d like to change (such as being able to default to a smaller trace size) if I can get development running again. So far after 2 or more years of effort, development is still not working (parts update on Windows is currently broken for instance, and can’t be fixed until we can get a development environment for all platforms working). Help in getting development going would be much appreciated by everyone.
Peter