I disagree with the previous post in that 20-30 lbs of muscle (one good offseason for you where you do everything right, training + nutrtition) will actually do a hell of a lot for your velocity, especially if you are already throwing hard for that weight. You could gain 60lbs and still be lean and strong as hell. Why would you wait to “fill out” (whatever that means - a guy like you probably won’t ever “fill out” without extreme changes) when you can make the change NOW.
I’m gonna disagree with kyleb on the mcdonalds for some of the reasons given, in that you don’t want to fall into bad habits that will stay with you when you’re older and may or may not have a speedy metabolism (not to mention other health risks). Go for it once in a while but eat as healthy as you can while still getting in at least 4,000 calories a day. TRACK YOUR CALORIES for a month like Kyle said and see what happens (assuming you are weight training 3-4 times a week HARD you should gain about 3 - 6 lbs of muscle with proper eating).
Cressey (and of course many others) have shown time and time and time again that adding quality muscle while addressing muscular imbalances, tissue quality, etc will not only improve athleticism, aid injury prevention and add a ton of power to your bat at the plate, but most importantly pack on MPH to your fastball.
A kid like you, 6 feet 140, lets say for the sake of argument you’re throwing low maybe mid 80s. 40-60lbs of muscle over the next 2-3 years will almost certainly have you in the low 90s. It’s your decision. If you want it bad enough, and are smart about it (See ericcressey.com for starters) it will happen.