Thanks to all
My real problem is not to enlarge and stretch traces, but to have good shaped large traces. So the problem is still here: there are no way to save a file with large traces and correct & "beautiful" joints because the trace's end seem to be too far from the centre of the pin/vias.
On the side of the code, unfortunately I cant' help practically, because I'm totally ignorant about coding, but I think that the "logical problem" of the code at the origin of the bad render can be the location of the round cap of the trace, that must be put on the same location of the pin/vias, and no more exotically calculated.
(BTW, why complicated rounded cap if a trace always terminate on a round ring/vias/pin? perhaps this buggy part of the code can be simply removed to fix all the problems...)
I hope some code-expert can fix this because like this Fritzing is totally unusable for PCB with large tracks.