Basically if you are gaining size and not mass that means you are either lacking calories or protein, or not doing a workout strong enough to produce the micro tears and re build.
The token amount of protein to have in building muscle is 1g per pound of body weight.
I weight 145 pounds, so to build muscle I have to aim for 145.
It is very hard to reach this hence many people who train don't grow as much as they would like.
Eg cup of milk has 8g, tuna is 18g etc.
That is good that you are taking the protein straight after training as after you lift weights there is a 20 monute "window" as they call it where the body acts as sponge and needs protein ASAP. hence a protein shake with about 30g after training is good. Use whey protein for this.
Now you know protein lets cover calories.
If you are quite a skinny guy it will be VERY hard for you to calculate how many calories you eat and I doubt you want to anyway...
Basically unless you have a far bit of body fat.. if you want to gain muscle you have to eat! eat every 3-4 hours with protein based meals. If you want to look at your calorie daily need though for boys is about 2000-3000 and girls about 1500-2200. you can put your name and age and everything into calculations in google and it will figure it out for you.
and with the lifting do it so on your 8th or 9th rep you COULD NOT do another rep for your life, feel the burn! and make sure you keep up with your water in take :) hope I could help!
If you already are lifting as hard as you can and going up weights every few weeks then your doing fine. when I am not strong enough to go up weights but want a killer workout. I will do this: (say its for biceps- eg bicep curl)..
start at absoulute max weight (for me is 7kg- im a girl)
I do 5 of them, or even 3 (whatever I can do) then without a break pick up a 5kg weight, do 10 reps then pick up a 3kg weight and do 15 reps. hence the musxcle is under tension a LONG time, and you have varied the rep/weighjt range for muscle confusion and lifted heavier than usual! its called super sets really good for muscle