Warm-Up or Mass Builder?
Should leg extension be done mainly as a warm-up movement for higher reps? Or worked heavier like other leg exercises? And do early in a leg workout, or later on?
Most do not consider leg extension a mass building movement for the quadriceps at all. Not really seeing anyone build bigger quads from only doing extensions. Muscle mass comes from compound leg exercises. Exercises like squats, leg presses and hack squats.
The greatest value is as a warm-up for compound movements. Even pre-exhaust exercise so you don’t need to go quite heavy on compound lifts. Even then, no point in going heavy on extensions. Doing so is tough on the knees and again, provides little benefit, if any, as far as stimulating muscle growth.
Stay with weights that allow 15-20 reps or even more at times. They can be done later in the workout if wanting focus on a certain part of the quads. Sometimes, turn toes out to focus on the vastus medialis, aka teardrops.