I've seen a few hundred P90X before/after shots from real people and didn't see much size growth. Skinny people got cut, which makes them look bigger and fitter. Doesn't mean they are heavier when they weigh in. Maybe added 5-10 lbs of muscle in a year. Full people got cut, also looks bigger and fitter in most cases. When you work out the whole body your bodyshape changes, shoulders get bigger so the perception is that you are bigger

A lot of hollywood actor training uses these principles. Actors need to get in shape quickly for various roles and trainers focus on getting the ripped P90X look instead of forcing real growth in such short timeframes. Ripped will look better naked than adding 10 lbs of non-ripped muscle. For example Bradd Pitt was maybe 160 lbs in Fight Club but looked much bigger because of muscle definition and upper body shape.

But if you really want to go from skinny to big/normal, not just ripped, heavy weights are the long term solution. 30 lbs of new muscle will look better than ripped. I don't know the details of P90X but recommended equipment is 50-90lb dumbells and a pullup bar. Most people use 50 lbs max and that's not going to cut it.