Also, a body weight increase includes increases in body fat and water retention, not only muscle mass. So your increase in actual muscle mass was probably 50% of what you think, again leading to a quite insignificant increase in metabolic rate by 150 calories/day.
You are assuming that I am like you. Its not about what I think, it is what I measure. I REGAINED that, meaning in 1999 on my high as a "natty" my bodyweight was 101kg with less then 9-10% Bf. On my hypogonadic low after years of trying to get my bodyweight back up I was 72kg and severely depressed. Almost a year ago I got on TRT and went back to the gym 6 days/week. Nowadays they have these fancy machines in gyms which measure BMI, water%, fat%, muscle %, endurance, strength, etc etc and I had regained in almost one years time on a variable dose of 17.5-28.8mg daily injected Test/DHT-combi, back to 93.3kg with a bf of 9% with 0 bloat. No broscience there as I temporary can not do my cardio-workouts and even with my high caloric intake, I still do not gain visceral or fat in nasty places, so no metabolic slowdown because of a high muscle/body ratio.
The 4.5lb of muscle mass increase was cited because it is the typical muscle mass increase of ordinary people on several months of strength training program in studies. Clearly for that group the increase in metabolic rate is insignificant and telling them to "just make more muscles to burn fat" is just a misguided bro-science.
You must understand we are not ordinary people, we are on TRT and have on a daily bases at least 3x the natural amount of exogenous testosterone flowing through our bodies. because of this I feel a strong drive and need actually to go to the gym, if I do not for a couple of days, I become narly and my wife makes me go.....lol Even without prior weight-training experience when you start TRT and working out at any age you will have a tremendous improvement in body-composition (although I apologise to those who have more serious underlying physical issues then just Low-T it will be less pronounced or non at all). Just look at some of the avatars of the peeps on this forum.....lol