ok with shorts like that you actually need the buttock group more than you need the thigh group, the reason its breaking off on the right is becouse the thigh group cant effect the hip group at all, so there is no influence of the thigh on the hip. personally I would group both sides all buttock.  as far as the effect in the rear that has to do with deft control of inner and outer falloff spheres right now you have the inner (green) controlling the area too strongly you need to back off on that a bit and make the outer (Red) blend the falloff area into the area that doesn't move a lot more. rigging pants in the crotch and buttock area is the trickiest part of rigging because you have so many groups overlapping each other as far as falloffs are concerned.