Doing high rep workouts with 10-15 pound dumbbells is essentially doing cardio. You will not build much strength, only increase conditioning. The reason you look more muscled is that you are losing weight and look more cut. If you want to actually build strength and/or muscle mass (they aren’t exactly the same thing) ditch the video routines, go to the gym and lift heavier weights. Barbells are excellent but you need some education about doing barbell movements (start with Starting Strength and/or liftbigeatbig.com for info), if you don’t want to do that, heavier dumbbells and kettle bells will do what it seem like you want. You want lower reps, higher weight.