Stretching is almost completely unnecessary. Contrary to popular belief you can't actually stretch a muscle fibre and lengthen it. Neuromuscular factors are involved with greater range of motion. Stretching after a hard lifting session can also worsen DOMS, so you get that tight and stiff feeling for longer. Note that the tightness and stiffness you feel is just that, a feeling. Not an objective measure.
Lifting "some" weights is fine. Taking it too far however and bulking up too much can be detrimental, especially with boxing being a weight limited sport. Canelo seems to have taken it too far.