Catalytic converter problem

So, I've removed the catalytic converter as a part of the power mods done to the car, and I can hear it burble when I let off the throttle or keep it very lightly pressed and decelerate, but no flames come out and no pops or bangs, (the ECU is stock).
I noticed a pipe that's wider than normal behind where I put the decat pipe, what is it?
photo for reference, the arrow is the flow of exhaust gases

So that would be the resonator, a quick google search - "The resonator is a complement to the muffler in that it removes high-pitched noises along with annoying hums and buzzes."

I would perhaps look at a better muffler or new muffler, to slightly increase the back pressure. Pipe diameters also play a role.

The muffler is good enough I think, I don't like back pressure, the car seems to run a lot better without restrictions.
also thanks for the answer!