Not to mention that it just spontaneously gains mass, without having apparently ingested it from any outside source. Why is it such a difficult concept to understand that mass has to actually come from somewhere - it doesn't just spontaneously appear.
I don't mind that. Aliens in Doctor Who do weird things. They time travel and miniaturise people and teleport and shapechange and regenerate and read minds and all sorts of things which are clearly sci-fantasy, not physics. This one gains mass. Not such a big stretch.