Granted, however while you can change the size of your breasts at will, changing other people's breasts will require the use of your breast milk. Every time you want to change someone, your breasts will start producing milk; the bigger your desire for change, the more milk you produce - though you can also partially limit or increase the production just by thinking about it.
By spraying the target with breastmilk, or by making them drink it (which is more effective), you will induce the changes that you want. More milk means faster changes, but obviously it also means making a bigger mess.
And that's a problem, because your milk can infuse other objects and living beings with BEE (Breast Expansion Energy) if you tried to make someone's breasts grow; and anti-BEE if you tried to shrink them instead.
If a mammal with breasts comes into contact with BEE, they will absorb that energy, and their boobs will grow as a result. If instead an inanimate object or an incompatible living being absorbs BEE, they will contain that energy and can spread it to other things; in this case, BEE fades over time, and each transfer consumes some BEE in the process.>For example, let's say that you end up soaking a field in 10 BEE. Plants grow in that field; now the dirt contains only 4 BEE, and the plants contain 4 BEE in total. If a mammal eats the plants or stays in the field for too long, their breasts will grow. Over time, the plants and the dirt will "decontaminate" themselves until all BEE will be lost.
Of course, anti-BEE makes breasts shrink. Combining BEE with anti-BEE will result in them canceling out each other, resulting in a small explosion of neutral milk.
I wish that this lobster monstergirl was my wife.