@Xulfiqar this is exactly what happened to me. I have been careless enough to leave my car with my mechanics a few times.
I gave my car to a local young ustaad recommended by my brother for break servicing and he ended up messing up the ABS. He attributed it to 'jugaar' done by someone else before him and recommended I replace the discs which I did. Long story short, it cost me 35k to circle back to a working ABS.
In the process, he cleaned the silencer as a courtesy and pulled the whole honey comb out. I didn't really listen to the exhaust sound until the next Morning in my porch (where it always sounds amplified) and went back to him. He showed me the whole packing and remnants of the converter which according to him was the 'kachra/gund' that came out of the silencer and which was making car heavy. He said, the sound is now more 'mature'. I discussed it with my brother only to find the guy had done the same to our other Mira too.
I am not freakish about consumption but the one time I have checked, it has gone down around 25-30%. I am going to start searching for a new cat converter as soon as I find some free time.