I just emulated the movement you outlined (re: sucking in)
What i realized was that my spinal erectors contracted immediately.
Seems like you have a muscular imbalance... primarily of the erectors.
You may also have compensatory postural issues.
Sucking your gut in forced you upright...
Probably rolled your shoulders back etc.
I'd work on strengthening your core altogether.
Primarily your erectors.
Also... you should probably work on your posture.
...and lose some more fat.
Excess weight normally exacerbates imbalances and weaknesses.
-CNS