did Apple changed the body of iPhone 6 Plus to avoid bending

Apple has not disclosed any alteration to either the iPhone 6 models.

The cause is mostly user caused (sitting on it).

From independent testing the iPhone 6 is more likely to bend than the iPhone 6 Plus. Please review this: Consumer Reports - Bendgate

Apple has announced that there will be a second hardware revision of the iPhone 6 to address other issues (flash storage). It would not surprise me to see undisclosed (perhaps internal) enhancements to the design. However, anything you can currently buy is the original design and I would not expect Apple to give credence to the

"bendgate" design flaws by announcing a fix.

My recommendation would be to invest in a rigid case if you use your device in a way that is prone to bending. Alternatively, there are plenty of people ready to buy your old one when you move to a different product. Apple isn't the only (or even the leader) in the ginormous fablet space anymore.