Do it longer but at a moderate level. Don’t overwork your muscles or you’ll build them. That feeling of your legs getting bulky probably isn’t them bulking up. You’re working your legs like you’re supposed to. You don’t want your moderate level to be easy because that doesn’t do anything for your body. You still have to push and feel the burn. Just as long as you’re not going at a high intensity to where you’re moving slowly but your pulse is skyrocketing. Then you’re working your muscles a lot. You want more endurance training instead of high intervals.