your hands can't hold as heavy of weight as your shoulders or traps can. therefore you are not lifting heavy enough with your legs on the squats to make gains for very long. about all that will do once you've built up to the weight of your heaviest dumbell will just be to tone going through the motions like a typical calastentic since u'll no longer be using more weight or enough weight to challenge the large strong muscle groups in the legs. the key to gaining mass is to overload muscles with more weight/stress than they can tolerate at their current mass. then have adequate nutrition and rest between workouts to allow the muscle to repair and then build up to adapt to the heavy load. you need to keep periodically increasing the load/intensity to get more growth because the muscle will adapt to what u do to it and have no reason to grow more unless u force it to do more. but remember the 4 keys, 1. workout with intensity and heavy weight. 2. maintain good form through everything in a workout 3. eat with proper nutrition and at least one gram of protein intake for every pound your body weighs. 4. give your body rest and time to repair and grow (as you workout longer and make more gains you will have to add additional days off between workouts as the more intense and heavier the workouts on an already developed body the more time that body will require to repair and grow too greater mass)(longer refers to over the course of time, weeks, months, years not the total time of a workout)(a workout with heavy weights should not exceed 47 minutes because then the body starts producing cortisol and your workout loses efficiency and results may be less and can lead to overtraining and injury)
