If it's just a cosmetic appearance that you are concerned of ....then u need to hit the gym and do legs workout as u do the upper body workouts...but if you genuinely have weakness of ur lower body....which can be caused because of Prolapsed intervertebral disc due to weight lifting or many other issues...then u need complete evaluation to diagnose if there is any pathology and treatment is needed to resolve that
Next Steps
visit nearest orthopaedic surgeon or consult to me via practo
Health Tips
avoid steroid medications for muscle gain