There's a lot going on with this picture. For one, the pose that she is doing is lifting up her breasts. It's subtle and not noticeable if you've never had breasts, but her nipple area is definitely not that perky and would not be straight onto the camera. She does this pose a LOT and it's a common one (hand/arm over the top of the start of the breasts) to make bigger boobs have less sag, because they naturally would not be taut like this. So with her tugging at the top of her breast tissue, it's obviously going to pull the veins to front and center. For some reason she also looks...sweaty in the first pic? Sweating/overheating can cause inflammation which would push veins closer to the surface.
As for medical reasons, there's a ton that cause veins to be more visible on the surface. Weight loss and gain, hormones, weightlifting. Hell, even just having big boobs in general because of gravity, it is quite natural/common. But with momo it looks so prominent cause she is so inconsistent with photoshopping them out that you tend to notice when she hasn't. Veins don't suddenly disappear and come back when they feel like it.
