Quantonium is a powerful radioactive energy that can fuel spaceships and turn humans into giants. It is the energy that turned Susan Murphy into Ginormica. It might be a liquid mineral mined on Gallaxhar's planet. As it is revealed in the movie that Gallaxhar destroyed his own planet, this likely means that the known supplies of quantonium have been exhausted. As demonstrated on Ginormicat!, It's effect on cats is that they grow until they're bigger than an entire planet.