to me it like classical electrodynamics ie. like Gauss's law connecting field on the closed surface with what is inside. The key in classical EM is that EM field is holomorphic, and the gravity in GM seems to be close to that too. So in that sense holographic principle is something like this - integral of information flux taken over closed surface equals the integral of the information [charges] taken over the enclosed volume, and thus the upper limit of r^2 is just being a simple consequence of the equality in the situation of maximum entropy (minimum information) situation of a black hole (with gravity being fully compatible with [and alternatively reformulated through] entropy description a black hole is the ultimate gravitational as well as ultimate entropy state).