I'm 5'4. After three years of the infantry getting my ass kicked by my platoon, working out at least three times a day, and eating as much protein as I could and taking the supplements I was allowed to take, I got to around 155-160.

At my height, 160 was the max I was allowed to weigh (more than a decade ago, hopefully the Corps has learned since now). The heaviest I was pre-deployment was around 165 (I had to "tape" out which basically means they just measured my neck and acknowledged my weight was muscle).

Once I came back from deployment, I was around 142-145 pounds, simply from walking from hours a day and eating MREs for months.

I had a similar experience to what you described my entire life pre-Marine Corps. I wrestled and did cross country going up and I was always frustrated that I was losing weight when I wanted to gain it. Once I joined the military, I was essentially forced to gain weight. The infantry is a pretty violent place and it was a mixture of forcing me to lift weights + eat six times a day and getting my ass kicked by 10-12 people (1-3 at a time) repeatedly until I understood that being smaller was a liability to me and the rest of my platoon.

Once I had the discipline (or probably obsession) with gaining weight, I had multiple alarms set. It looked like this (specific work outs changed throughout the week):

1. ⁠5:30 AM: Wake up, drink water, eat three spoonfuls of peanut butter.
2. ⁠5:45AM: Stretch for 15 minutes, then go to 6:15 AM morning formation.
3. ⁠6:30-6:45ish (depending on the day. Lots of empty time for junior enlisted in the infantry when you're not deployed; my chain of command pretty much told me to normalize the 7:00-11:45a list). Sprint to the gym (less than a 1/2 mile from my barracks. I looked at this as training for the "movement to contact" portion of the CFT).
4. ⁠Lift weights until 7:45 alarm. Shower, PTRT (PT recovery time).
5. ⁠8:30 alarm: Eat six eggs, three spoonfuls of peanut butter, and a glass of whole milk.
6. ⁠9:00 alarm: Walk outside and do max pull-ups at the bar 100 feet away.

From 9:15 to 11:45 would vary by day. Sometimes I'd have to be at the armory cleaning weapons, other times at the motor pool to clean/do lowest level grunt maintenance on our trucks. But if I didn't have to do these things, then I'd go back to the gym from 9:15-10:15 and continue to lift weights, this time a different portion of the body.

6) 10:30 alarm: A can of tuna fish mixed with peanut butter (trust me on this one) and more whole milk.

7) 12:30 alarm: Eat whatever is at the chow hall. If ice cream is available, I hit that section twice.

8) 2:30 alarm: another protein heavy meal

9) Between 4:30 and 5:45, depending on when the Company was released for the day, I'd eat another protein heavy meal and max out pull ups again.

10). 6:30 Alarm: Three spoonfuls of peanut butter and run to the pool (a mile away)

11) I'd swim until 7:30, leave the pool by 8:00 and eat a protein bar, some fruit, and drink water on the way back from the pool. Most days I'd also eat a sandwich during this time as well or eat more eggs. If the pool was packed/closed I'd go back to the gym and lift again.

12) 9:00. Protein shake with whole milk, eat some more peanut butter, go to sleep.

I had to get to a point where I ate more than I was burning off working out.

Unrelated fun fact: I've seen more grunts get kicked out for [DAT DERE] (then [PEPSI] as a second place) than any other drugs.