Obviously your triceps training are wrong if you say that you have small triceps. Logic no?

If you want mass, then you need to train for 8-12 reps range. I don't say that 200lbs triceps pushdown isn't building muscle mass for you but what's the purpose of going 5 reps on triceps push down? It's not compound like squat or deadlifts.

Go lower weight with the pushdown and up the reps. 3x8-12 should be good. Then, do some skullcrusher / french press albeit some people will find this hard on the shoulder. Triceps kickback is a good exercises. If you want and able to go heavy, do weighted dips (normal dips, not chest dips) and make sure it reps range are high.

Let's see the ratio here. You can do triceps pushdown for 200lbsx5 and triceps dumbbell extension for 35lbsx15. You can see the huge weight margin here yet you train the same muscle. Shoulder might come in play when doing TDE. If you are doing triceps pushdown using bad form where you engage your triceps, shoulder, chest (and ultimately turning it into a press movement) or very short ROM, you won't be isolating your triceps hence the 200lbsx5 pushdown compared to 35lbsx15 TCE.

So, by that logic, I suspect you've done your triceps pushdown with incorrect form and didn't isolate your triceps during that exercise.