It's not even the boobs; if you check the size of her head with the entire body, her entire body is about 8⅓ heads tall; most reasonably sized "adult" humanoids tend to be in between 8-7 heads tall, and going outside of that range makes the character either like a chibikko or a giant.
Loli Cirno has about 5⅔ heads of height, so her proportions don't look too bad for a loli.