I have 4 tattoos - between my shoulders, bicep, ankle, and breast.
I have lost and gained ALOT of weight in my adult life, and had 3 kids. The one on my boob is the only one that has changed.
I got it when I was 17. It's a cluster of 3 roses, and it's my smallest one...only about the size of a silver dollar. When I got it is had a circular shape. I am 32 years old now, and it is more like an oval now lol. It looks like it is sliding down my boob at an angle or something. It used to be straight like this - now it's more like \
When I have on a good bra you can't really tell, but the girls just aren't what they used to be
I can't say for sure whether it's just the weight loss and gain, because I've also been pregnant and breastfed several times since then.
If I had to bet on it I'd say that in most areas of your body it won't have that much of an effect, but some parts are just prone to sagging as we age, weight aside. Skin sag is more the issue than weight itself, because it permanently distorts the shape. Gravity is the devil
If you have huge boobs like me, getting a tattoo there just isn't a good idea. It is the only one I regret.