WOW 155 at 6'3" damn man you don't need 2nd helpings...you need a truckload of food every day to get to an appropriate weight...
Okay all kidding aside, they probably used they BMI chart to see if you weight was "correct" for your height, which i think you and I both agree is way to low, but on the other hand the BMI chart is a REALLY bad method to evaluate this, and was never meant for that purpose.
because if you took my BMI it would probably be something like; high 20s low 30s maybe, im 5'8 and 210lbs. with a 34inch waist...so I am not fat by any means, but according to this chart im overweight-obese, and i should lose around 40-80lbs, which is NOT going to happen if anything I will continue to get heavier and bigger, to approximately 250-260lbs offseason...so I will always be termed obese according to the BMI.
For your height according the BMI you should be anywhere from 150-190 is "normal range" but honestly 150 at 6'3 means 1 of 2 things...you're either a lanky skinny long distance athlete, or you live in a third world country where you get food once a day and thats on a good day.
When it comes to weight its individual, partly how you like to look, and partly your build. By that i mean are you naturally a big guy (not fat) but big...aka broad shouldered, big arms etc, or are you naturally match stick thin.
Personally i started off match stick thin at 130lbs when i was 14...nearly 8yrs later im 210, and thats not making me happy lol.
Basically what i am saying is that if you feel comfortable at 155 being 6'3 then fine go for it, besides my mom joining the army back in the 70's i have never heard of anyone being told to gain weight (she's 5'3 and was 90lbs) and believe me on my BMQ, i had plenty of "twigs" aka tall skinny guys averaging 6' and up but not weighing more then 150-160, so i don't think they wouldnt let you in if you were not heavier.
Side note: personally i dont think you should weigh less then 200lbs anytime you are 6' or more unless you are a marathon runner, or something like that, because you miss out on a lot of potential strength that can be very useful and for you at 6'3 obviously depending on your bone structure and what not i would say a good healthy athletic weight would be 200-220.
As far as meals on BMQ, well only 3 squares, which i never understood why EVERYONE does that, its clinically/scientifically proven to be better for you to ingest smaller more frequent meals. Back to BMQ though yes sadly only 3 squares, and as for seconds yes you can, but only if you have the time, your MS/Mcpl will say "its now 12:10 i want everyone back here formed up in front of the shacks at 12:50, Platoon dis-missed." That means you have 45mins for lunch break, meaning you have 45mins to get your butt to the mess, wait in line get your food it (go a 2nd and 3rd time if you can), hit the heads, shine your boots or whatever you need to do and then be formed back up 5mins before the time MS/Mcpl said to.
Be careful though we had an a$$ hat on our course, who screwed our lunch up for us once. When we were all in the middle of eating he whined about something and one of instructors heard him they said you have 5 more mins to eat now get your butts out there for remedial PT (we got 10 mins to eat total tha lunch).
OH maybe something that can help you, we (several of us on our BMQ) wrote our Platoon 2IC in standard memo, to ask if we could purchase protein powders and cups and keep them in our rooms or with the Barrack warden, and we all got the OK, so maybe you could look into that, although im not positive all the instructors are that enlightened.
Best of luck and enjoy every minute