The way you worded the question it could be either way. If balloon size is a function of amount of helium, balloon size is dependent on the independent amount of helium, whereas if the amount of helium is a function of balloon size, then the amount of helium is dependent on the independent balloon size. If you already have a graph, the one on the vertical axis is dependent on the variable on the horizontal axis.
John
My calculator said it, I believe it, that settles it