I have a spritesheet (packed by the wonderful TexturePacker) that looks like so:

and the animation as an animation works well using the current code. However, when the animation moves to a frame where the bounding box is smaller (e.g. when the legs are closer, when the figure is kneeling, etc.), the image distorts so that the smaller bounding box fills the bounding box of the sprite.
How can I fix this?
The spritesheet above is a frame-extraction from a sample Adobe flash animation.
Easier way: You can make all the textureregions the same size.
Hard way: Keep it like this, but you need to save and calculate the size and position of every textureregion to draw.
I personally recommend you the first option, just make all the image the same size.