Although I was consulted for Well+Good's article Does Spinning Lead to Bulky Quads, I feel that my answer was a bit sugar coated to read as pro-spin propaganda. My honest answer may shock and confuse you. Does spin make your legs bigger? I say...YES. I have been spinning for 8+ years now, turning the resistance up every time and my legs are just enormous. I wear a size 4 jeans now! a 4! Wait. I'm the smallest I've been in years. How can this be if spinning makes legs bigger? Let me explain: Initially, my legs were not as strong. I began spinning and building muscle in my legs. A pound of muscle burns up to nine times the calories of a pound of fat. I continued eating normally. No diets. No restrictions, but knowingly getting enough fresh vegetables- and eating salty/fatty/sugary foods in moderation. Here is a brief history of MY BODY ON SPIN:
1 month of spinning: I was spinning 2x/wk. My legs had gained more tone and muscular shape. Yes, they were bigger. I weighed 135lbs and wore a size 6. My jeans felt a little tight in the quad but I didn't "outgrow" them.
1 year: I am still spinning 2/wk but now doing "doubles": 120min instead of 45min. I'm also riding a real bike outdoors at a casual pace almost daily for commuting purposes. I lost 4lbs and I'm pretty sure I was eating more.
5 years: Legs are smaller now. I know this because the jeans that used to be tight are now comfortable. I'm spinning 4x/wk, doubles. My weight is at a record low of 128 (for a 5'3" girl that seems hefty, but I'm wearing a 5 in Juniors jeans- which leads me to believe most of my weight is muscle.)
8 years: NOW my body weight is at an all-time low of 124. I spin 6 days/wk plus ride a bike around the city everywhere I go. I am wearing a size 4 jean, and comfortably fitting into size 3-5 juniors. Most of my weight IS muscle. My body fat is 15%. I'm still eating the same: No diets. No restrictions. But including more veggies.
The only people who should be concerned about jumping on a bike and turning up the resistance are non-athletic supermodels and ballet dancers who have a career in being thin and cannot afford to lose any weight- nor can their career allow more developed muscle on their legs. Unless you fall into this category--- get on a bike, turn it up, and sweat it out!!
Does this bike make me look fat? No, it makes you look FIERCE.