How much stiffer are 23c tires over 28c?
I assume by "stiffness" we are referring to comfort, and that the Kenda will have a cheaper tire construction. In that case which tire is more comfortable depends on how you set up the tire and your riding conditions. If you are riding on very smooth roads, then a properly set up 23 mm GP 4000 may actually be more comfortable due to the combination of a  higher quality carcass construction and the smooth roads not requiring large pressures to avoid bottoming out the rim.  If you ride on very rough roads or mixed terrain then the 28mm Kenda's may be more comfortable, as you can run a lower pressure than the GP 4000, which will better absorb impacts therefore feeling more comfortable and less "stiff."

Background

"Stiffness" relates to tire construction and casing tension (i.e., tire pressure).  Cheaper tire construction tends to result in a stiffer casing and therefore will feel "stiffer" for a given volume.  The continental GP 4000 will have a higher quality construction than the Kenda, therefore if all things were equal (e.g., size and pressure) the Continental would actually  be less "stiff."  The Kenda is also larger, and a larger volume tire will actually have more casing tension and therefore be "stiffer" for the same tire pressure, but these two tires shouldn't be run at the same pressure, which makes the comparison more nuanced.

So Why Do Larger Tires Have a Reputation for Being More Comfortable?

The key issue is that all things are not equal, including pressure.  The main advantage of larger volume tires is that you can typically run them at a lower pressure than smaller volume tires without risking issues such as bottoming out the rim. If the roads are rough enough, the comfort gains from being able to run a lower pressure  can out weigh any differences in carcass construction.

As such, we need to consider the environment you will be riding in.  

If you are on rough roads you may not be able to lower the pressure enough in a smaller volume tire (e.g., 23 mm) to achieve the same level of comfort as with a larger volume of tire (e.g., 28 mm) which can be run a lower pressure for the same level of rim protection.  Because you will need to maintain a relatively higher pressure in the smaller tire to avoid pinch flats or damaging your rim, the tire can often ride more harshly (i.e., feel "stiffer") under these conditions. 

If you ride on very smooth roads then a 23 mm tire can be run at relatively lower pressure, which will provide more comfort in addition to there being smaller bumps to absorb. In this scenario it will therefore not feel as "stiff,"  in fact it may even feel more comfortable than the larger Kenda as the casing will be more compliant than the Kenda tire casing adding to the ride quality.  

Interplay Between Tire Pressure, Tire Size, Road Conditions and Comfort

The real key is whether or not the tire volume is sufficient for the the road conditions.  If the volume is too small, you are likely to hit what has been termed the pressure "break point", where suddenly more vibrations will be transmitted to you, which can be measured as an increase in real world rolling resistance (Figure 1).



Figure 1. Real world example break point pressure measured by Tom Anhalt from Part 4b Rolling Resistance and Impedance. Red arrows and text were added to the original figure for the purpose of clarity. 

Tom Anhalt nicely demonstrated that an increase in tire pressure lead to less rolling resistance (as predicted by theory), until suddenly it didn't, because the pressure was too high for the tire to effectively absorb road imperfections. This resulted in increased vibrations transmitted to the rider, moving the rider up and down, which absorbed forward momentum, which was measured as higher real world rolling resistance.  

From a comfort stand point the rider would likely interpret the extra vibrations as a lack of comfort from the tire being too "stiff."

Lowering the pressure below the "break point" will return comfort and the tire feeling "less stiff."  This assumes however you can safely do so.  If the roads are too rough, then you will start bottoming out the rim and risk pinch flats.  As such you will need again raise the pressure to keep things safe, if this increase in pressure sends you over the "break point," then suddenly the tire will feel too "stiff" due to the extra vibrations begin transmitted to the rider.

Predicting where the break point lies in real life is difficult.  Tire construction and size that will work best for your application will likely involve trial and error.  That said, experimenting with pressure can be an important first step. Your loading weight (i.e., rider + bike + load) also is an important consideration as a higher load weight requires a higher pressure to ensure the rim doesn't bottom out.