As the above question stated, the upper and lower section of the combustor isn't same. Is there any reason behind it? Due to which the cooling of the combustor is also different in the upper and lower section of the combustor. The below picture clearly depicts it:
That drawing in the question is not really representative, here's a CFM56 combustor:
The bulge the artist used, is actually a turn/inclination for aligning the combustor with the more outboard first stage turbine.
Here's a more gentle turn in a bigger engine: