But surely we have oodles of data from devices like Fitbits and Apple Watches to know whether this behaviour is true for humans or not.
exercisers, whatever their species, tend to become hungrier and consume more calories after physical activity. They also may grow more sedentary outside of exercise sessions. Together or separately, these changes could compensate for the extra energy used during exercise, meaning that, over all, energy expenditure doesn’t change and a person’s or rodent’s weight remains stubbornly the same.
Looking back a few years later, I realized maybe I didn't see the big changes I was expecting since my roommate and I would hit the Burger King on the way home since we were always so hungry after working out. It dawned on me we were completely erasing all the work we just did in the matter of twenty minutes eating cheeseburgers and onion rings with several cups of pop to boot.
It's also hard to gain muscular physiques in general. You have to cut an enormous amount of body fat to obtain things like an Adonis belt but if your goal is strength it shouldn't be too hard to hit novice benchmarks.
I also doubt you were seriously burning the amount of calories you think you were working out. Unless you're a professional or Olympic athlete, someone who is training for over 6 hours a day, you will likely never burn more than 400 calories from a "moderate" gym session.
People dramatically over estimate how much they burn working out. There's a reason why people say "You can't outrun a bad diet."
Not to mention many people spend just a fraction of their time in the gym actually lifting weights. Mostly sitting at machines, resting between sets and playing with their phone.
My gym routine is "super-sets" pairs of exercises working complementary groups of muscles. So I'll do say 4 sets of lat pulldowns alternating with shoulder presses, with zero breaks. Then switch immediately to the next pair of exercises.... Net result is far higher average intensity in a shorter time.