Quantonium is a powerful radioactive energy. It is the same 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!, its effect on cats is that they grow until they're bigger than Earth.