Originally Posted by Trevor Ross
That depends on the grip width of the barbell and if there's scapular retraction, sure your triceps are the primary movers, but there are methods to put emphasis on the muscles you intend on working.
It has to do with the physiology of the movement: your arms are doing the pushing. The pectoral muscles' primary purpose is to pull the arms down and across your torso. The way your pectorals are stressed during the movement is towards the bottom position; when they are in a full stretch.
It is impossible to push your arms upwards from a pronated position without pectoral involvement, hence they are abductors for your arms. However, your chest is not actually lifting
anything; your Triceps and shoulders are. This is why Tricep strength is so often stressed when training your Bench.
If you really want to stress your chest muscles, try doing a variant that gives them as full of a stretch as possible in the bottom position. You will get a big chest, but that's not because your chest was doing a lot of pushing.