Also I have a friend who already has huge tits, and I’ve seen them IRL so I’m curious what it would do
Being serious for a moment, it depends on the source image. If it can tell where the contours of the tits are in the source image, they'll be closer to the right size and shape - otherwise it's going to find something it thinks are the contours and map out tits that match those, then generic torso that matches the shape of where it thinks the torso is and skintone of the face. It's not magic, it's just automating what a horndog with photoshop, a photo of you and a big enough porn collection to find someone with a similar body type could do back in the 90s.