No you pretty much have it. A bigger tarp with ground sheet will be approximately same weight as bivy with small tarp. The extras you get with bivy are warmer...extra layer around you to trap warm air, blocks wind, provides bug protection, can be used on its own, keeps mice off you, and provides sense of security. Being wrapped inside a bivy when a big storm blows through is just fun for me. Lastly a smaller tarp/bivy requires less space to setup.
I'd say the advantage of a larger tarp is flat out more living space. There is more room to cook, more room for gear, easier to sit up, etc. Since I don't spend much time under my tarp this doesn't do a lot for me. If I did then a larger tarp would indeed be a nice advantage. You might also consider that condensation management is easier since you aren't dealing with the trapped moisture in a bivy.
I think both techniques are valid approaches.