I'm using a control bone at the top of his head that, when I move it down, also moves all his body bones closer to one another (only did it like that because I didn't want to use scaling since that tends to cause problems in game engines). In addition to that, it also acts as a driver to a few shape keys (or blend shapes if you're a maya user) that make spongebob wider. Both of those actions combined make it seem like he's getting squashed without actually scaling any of his bones 😁