Two reasons.
I don't care what you say, you aren't eating enough. If you eat good, healthy food including plenty of fruit and veg as well as things like good quality fish, meat and eggs, you will get strong before you get fat. Vegetables are the forgotten supplement. Take daily in large doses.
Secondly stop doing pussy, isolation-heavy, bodypart-split routines. For beginners, size and strength are directly correlated, so you will not get bigger without getting stronger and vice versa! You need to do a full-body routine which focusses on regular strength progression in big, compound lifts. It's good that you do things like squats at all, too many don't, but they aren't just there for a pump. If you manage to increase your max weights across things like squats, deadlifts, bench press, overhead press, rows and chin-ups, you 100% will be bigger. I cannot stress enough how much you should focus on strength first, even if it isn't your aim. I recommend checking out Starting Strength, Stronglifts, or Westside for Skinny Bastards 4, they have all helped infinitely more beginners put on size than any of the routines you see on BB.com or equivalent.