Acoustic guitar bulge behind bridge

A little bit of a bulge behind the bridge is normal and fine. If the bulge gets pronounced enough that the saddle begins to lean towards the bridge, bulge looks uneven, or the bridge starts separating from the body then you have a serious problem. A smooth even bulge is no big deal.

1. What causes belly bulge in acoustic guitar?
2. Should the back of an acoustic guitar be flat?

What causes belly bulge in acoustic guitar?

Sometimes acoustic guitars develop a bit of a belly. Over time the string tension pulls on the bridge, angling it forward and warping the top, which in turn raises the action and causes other structural problems down the line. ... To fix this we have to remove the bridge and address the top directly.

Should the back of an acoustic guitar be flat?

no, not necessarily. A large (slight) radius like 24-30 foot is common. Structurally its more sound with a slight curve.