Yes, and once you also 'load on' the a/c - c/c then a smaller engine uses juice at an even faster rate! 
Not totally in agreement here.
What you have to consider is that a set of amount of energy is required to accelerate the vehicle weight, overcome wind resistance and operate the ancillaries and this is very much engine independent (excluding slight weight differences between the different power plants).
This amount of energy will relate to a set amount of fuel, the quantity will only vary based on the efficiency of the engine at the particular operating point.
And as we all know, more cc gives a flatter torque curve which gives a wider operating range and hence why the fuel consumption of the two aforementioned power plants will be similar despite the tendency to boot the V6 a little more (it is capable of using more fuel at peek operating conditions 
)
I think I know what you are getting at Mark, but I would argue my point in a simpler way:
In the USA we hired a Volvo (2.0ltr) and when we used the a/c the fuel needle fell dramatically fast, but when a/c turned off it did not.
Then we hired a 5.5ltr Ford Crown Prince Victoria, and using the a/c or not made absolutely no difference to fuel consumption.
I have found exactly the same situation with 2ltrs V. 3ltrs in the UK; in the former the fuel needle drops fast with a/c on, but in the 3 ltr it made little difference.
The moral of the fact is that the bigger capacity of the engine the smaller the a/c drain of power represents on the overall power output of the engine. Thus on a big 3.0 ltr engine a/c may represent, say 10% power output drain, but on a 1.6 ltr engine it represents 18%. Thus petrol consumption will be greater on the 1.6 when the a/c is on as oppossed to the 3ltr.
Think your over looking the rules of energy use....
....it requires the same amount of energy to push a car forward (of the same type/weight) and if you turn the aircon on it adds an additional constant load.
Now the fact you have a fairly light weight car in the form of a Volvo means that fuel consumption increases with aircon on might be noticeable.....when compared to a heavy weight with a great iron V8 in becuse the percentage of pwoer being consumed by the aircon might be smaller on the bigheavy V8 car (not a good comparison at all as they should really be the same type of car!).
To meet those energy needs we are burning fuel and this requirement will be pretty constant between say a 2.5V6 and 2.0 petrol Omega with or without aircon. The only difference is that because the V6 will tend to have a wider and flatter torque curve that you are more likelty to be running at a sweet spot in the pwoer delivery curve.
But surely Mark that is the point the greater power to weight ratio the easier it is for the engine to cope with the (constant and universal) loading of the a/c. :-/ :-/
I do understand what you are saying though; the principle of relativity (Newton I believe). But is it not a fact that it is far more efficient / economical to fit an engine that provides more than enough power to push the machine (plane, car, train, etc) forward against the resistance of air, tar mac, metal, etc) than fitting an under-powered unit? Therefore a 3 ltr in the same car as a 2 ltr must be more effecient and therefore superior in coping with the loadings. :-/ :-/ :-/
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Topic: Is a 2.5 more economical than a 2.0? (Read 3964 times)