531Aussie

Here's something I've observed over the years: we've all heard that endurance activity can strip muscle. However, the pros, who do billions of miles a season, seem to maintain the same sized legs year after year, unless they deliberately drop a stack of weight, like Wiggins and Froome.

Ok, we can easily conclude that drugs helps them keep muscle, but still... it's not like the 30,000km/year they ride makes their legs thinner and thinner through their careers

OBoile

Please keep in mind that pro cyclists tend to have pretty skinny legs... they just look bigger because their upper body is even smaller.

But you're right overall IMO.

A caloric deficit causes your body to lose muscle. How much muscle lost (and where that muscle is lost) vs how much fat is determined by how your body perceives the need to retain a given muscle. Lifting heavy things and/or doing short bursts of intensity send a stronger signal to your body about the need to build/maintain muscle than long endurance work does. But, as evidenced by pros' proportionally larger lower bodies, even endurance work does drive some muscle retention. A sedentary person who starved themselves to that level of bodyfat would have very skinny legs to match their very skinny upper body.