Getting stronger but not much bigger?

hey!

I bought dumbells and started lifting in September to get my arms stronger. I started off managing only 15 reps with 5 kilos! I got much stronger, moving to do the same amount but on 10. I then wanted to work my whole body so i joined the gym which i go to every 2 days at a steady routine and i have to say i like the results but i am not too impressed. I am 6 foot 17 years old and only have about 12 per cent body. I do about 2 miles running at the gym, 5 sets of bicep curls at 14kg, i work all my upper body, stomach etc. and my legs for about 80 mins. Eventhough i am getting much stronger in a small amount of time which i hoped for and a 6 pack is showing, i arent really getting that much bigger. I mean, after every workout i eat a healthy meals such as fish with lots of protein and 1 hur before bed a can of tuna. I also only drink water now and dont eat crap like sweets and fatty foods. My muscles have become more defined and such but i dont understand how i have become so much stronger without gaining much muscles. I in fact weigh less than i did 6 months ago. I dont know if i just dont notice the changes because i see myself every or if i am gaining muscles and losing fat which balances out or what.

any ideas on what the problem as and if this is normal? thanks