Ditto to @TylerJamesYoung's answer, just to add:
"To mushroom" is an idiom -- well, probably more of a metaphor -- meaning to grow or expand very rapidly. We often say, "Sales mushroomed after the new advertising campaign" or "World population mushroomed when the Green Revolution increased crop yields".
Note this is quite different from the meaning Tyler refers to, "to assume a shape like a mushroom".
Some people refer to hollow-point and soft-point bullets as "expanding" when they hit the target, but this isn't really true. They don't get bigger: they flatten out. The diameter gets bigger while the length gets shorter. It's not inflating or magically increasing mass.
So in general if you said that something "expands and mushrooms", I'd understand you to mean that it grows larger. But in this case, it means that the diameter gets bigger and it takes on somewhat the shape of a mushroom.