What are some possible reasons that my veins look so big?

Large veins. Veins can be very prominent and still entirely normal, like veins on the backs of the hands and the tops of the feet due to lack of surrounding soft tissue and underlying bone. Weight lifters and runners can have very prominent veins because of the extra blood that is carried to and from muscle that is being exercised and hot. On the other hand, varicose veins can also be very large too.