So since the OP got completely checkmated by this chick and his mind is off in la-la land somewhere, the lesson for everyone else is:
What caused this to happen? And how could I prevent it from happening to me?
Something to think about.
My answer would be, as it usually is: always have multiple relationships going at the same time. Imagine what would have happened if OP weren't dependent on his one fat girl for all his social / sexual / psychological needs. And NB: you can't wait until the breakup happens to start dating other chicks and expect that to work. Too late by then, you need multiple established relationships for them to help when you have a crisis with one chick. Also, they tend to prevent these crises from happening in the first place.
The second thing I see is that OP got suckered into depending greatly on her in many other ways... phone, TV, furniture, dogs, etc. etc. etc... all were hers. She knew that withdrawing not only herself but all her stuff would show him how lost he is without her. Brilliant move on her part. I'll give her that.
She basically said, "Look around you, you are nothing without me, and you have nothing without me." He looked around at his empty place and his dwindling Facebook friends list and said, "You know what, you're right."
A move so forceful and which takes so much energy and causes distress to so many people (including families and friends) has one end goal: marriage.
EDIT:
I find it absolutely fascinating the OP originally stated that the chick gained "80 lbs" and then, once he allowed his world to be destroyed by her, changed that to "40 lbs."
Best thread on SS in months. So eye-opening with great lessons taught. Really a good one.
EDIT 2:
The only problem I see with her immense power-move is that she allowed him back in too quickly. This might prove to be the downfall of her plan. For her move to really work, she should have held out longer. Make him grovel, make him really beg. Flowers, more dinners, and within a month an engagement ring. That should have been her play. She buckled too easily, and that fact might save our OP.