I have noticed fungi and mushrooms tend to use less bone meal than saplings. This is probably because they have no second stage of growth that saplings do have. This doesn't seem intentional as one of the 1.5 updates nerfed bone meal significantly, but inconsistently.
After some testing, it took an average of roughly 1 1/2 bone meal per mushroom, and over 3 bone meal per sapling
I am not asking that a second stage of growth is added, I am just noting there is an inconsistency, and that it could be dealt with by balancing probabilities of growth for either or both saplings or mushrooms