Oh my, she already had impressive mana tanks, now they are even bigger.

Squish !

What does a mana crystal do?

Mana crystals are essentially the excess mana leaving the planet's magical leylines and are mana in condensed, purest form. So pure, that they will suffuse water to create mana baths (used to recover the people bathing within of their lost mana and increase their natural capacity slowly as they soak in the liquid, as well a treat ailments like mana burn), or get fashioned in small quantities within trinkets or outfits to boost the wearer's capacity while worn.
However, if a mana crystal comes into direct contact with the skin, the condensed mana quickly starts to infuse into the body; as a result, the body tends to change accordingly to house the excess energy, within only a few minutes.

In my setting, girls' mana capacity is directly linked to their chest size ("Mana is stored in the boobs!"), so the higher the capacity, the larger their breast size (within reason), usually. A high influx of mana via a mana bath or a mana crystal (or semen), will thus cause their breasts to become bigger, in accordance to how much excess mana they store, and shrink as it gets used up again. Using too much mana can equally cause their breasts to temporarily shrink if they run low, until they eat/sleep/drink or take in external mana to replenish their reserves.

So the bigger da booba, the stronger the magic?

In effect, yes. Having more mana means you can cast stronger versions of spells, or more of them.

Wow looks good and cute ✨