wait...the only exercise you're doing for triceps is a press on a multigym?
ummm....you can't get bigger with only one exercise, especially with a complex muscle group like triceps, or biceps for that matter.
also, 'step away from the machine.'
cables and machine exercises are great, but usually only after you've done some heavy work with free weights...
now, having said that, there is always an exception, in this case it's pressdowns on the cable machine, which you can do heavy with varying grips, as part of your initial workout...however, since you've already been doing them, I suggest you leave them out for a few weeks and focus on the newer exercises these guys have listed above. your workouts should change every month or so, so your body doesn't have time to adjust.
good luck...13' arms, the only place to go is up.