A thigh tattoo will only stretch out enough to be seen if you gain a lot of muscle or weight. Skin is pretty flexible, so it will take a lot to change the shape of your tattoo. If you start lifting weights, your tattoo might change slightly as your muscles get bigger, but you probably won't notice. When you gain weight, stretch marks may be the most noticeable change. Again, unless it's a big change, you won't really notice stretch marks, but the texture of your design will change a little.