bcql Ah, I see what you mean, but this is an expected behavior. The GIF you have framed in red looks like this:
Since the Spineboy falls backward in the animation, the size that fits its entire movement is recognized as the appropriate boundary.
So if you want to export at a specific size, you can either use Crop as recommended by Spinebot, or you can create a clipping attachment in the skeleton and use it to crop the entire skeleton for export.
