Big Chicken vs Small Chicken: weight and stuffing volume ratios
I think you are over thinking this...

One one hand, the size of the cavity in the chicken is approximately proportional to the width or height of the chicken, and the width is approximately proportional to the cube root of the weight.  However, for small values, the because of the cube law, the volume of the cavity is not going to change by huge amounts--a 2.5 kg chicken will take almost as much stuffing as a 3 kg chicken. 

From 2 to 3 kg, you are looking at a ratio of approximately 1.4 versus 1.7, or very approximately 80 percent as much stuffing.  For most cooks, this is in the noise, I would suspect.

However, speaking culinary:


You can simply look at your chicken and estimate the volume of the cavity, or stuff it until it is full--see item 2.
You can bake any excess stuffing separately in a casserole dish.  Or you can bake all of the stuffing (technically, now dressing) in a casserole, which is my preferred method--then the amount you want is directly proportional to the number of guests you have.*
Many folks, including myself, believe roasting the chicken (or turkey) without stuffing leads to better meat, is easier, and the chicken will certainly cook more quickly.
Lots of folks like the stuffing the best, so your guests might want lots.
Leftover stuffing is delicious, and can be frozen.


*Geeks may point out that the guests may have to be weighted by a hunger factor...