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.
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Topic: Is a 2.5 more economical than a 2.0? (Read 3970 times)