Isn’t this against the laws of physics? If you’re burning a ton of energy and not eating excessively, where are their bodies getting mass from to maintain weight?
Eating 1000 kcal more a day is pretty easy to do without noticing. Especially when you eat a lot on the day that you biked (cause "you earned it").
For example 2 liter bottle of coke is 800 kcal and 1 protein bar is about 250 kcal and you're already over the budget (assuming you also eat your "regular" food).
No because they were also eating a lot, the lower bound on this established by physics.
I've known a lot of roadies who sort-of-correctly believed they could eat as much as they could physically manage, as they were expending enough (3+ hour intense riding daily) that it was a chore to keep up. At least when they were 20-something and racing. Many struggle a bit when the a) get a bit older/metabolically slower and b) life gets in the way and they ride a lot less. All of a sudden a guy who was rail thin gains 30lb that "sticks".
So really it's that they weren't losing weight because they were eating too much... exactly like most people in modern western society. And really the moderate amount of cycling was irrelevant.