The problem is that if you increase the size of your Image Target, then in a certain way, QCAR will adjust the "world size" with respect to your camera;
if you like, you can think of it as if you are changing the unit of measure in your world;
for example, imagine the original size in the samples (width = 247) is in millimeters (which is actually the size of an A4 paper);
then, when you hold your printeed target in front of your camea at (for instance) about half a meter distance, Vuforia will detect it and will report (in the pose) that it is about 500 units away (in practice, 500 millimeters, right?)
Now, suppose you change the target size to 24.7 (i.e. 10 times smaller); you can think of it as if you are now using centimeters instead of millimeters (247 millm is = 24.7 cm).
Now when you hold your printed target still at half a meter away from the camera, Vuforia will report (in the pose) that the target is about 50 units away.
So, to summarize, we have reduced the size by a factor 10, and thus we get a distance from the target also reduced by 10 (from 500 down to 50);
now, at this point you want to render your Teapot, but the teapot mesh (vertices in the vertex array) is defined (suppose) in millimeters
(i.e. you have vertex coordinates that make sense for a real teapot if you interpret those coordinates to be in millimeters);
because the coordinates of the teapot are in millimeters, they will range between for example -100 and +100 (just as an example) in X, Y, and Z;
so suddenly, when using the size of your target in centimeters, your teapot will now look huge, because Vuforia will place it at "just" 50 units (instead of 500 units) away from the camera;
In short: reducing the size of the Image Target (in the XML) has the consequence of increasing the size of the teapot (assuming you are using the same A4 printed target in all tests);
and vice versa, of course, increasing the size of the target (as you did), results in the teapot to look smaller, thus requiring a larger scale.
So, it is normal that you have to increase the kObjectScale in your case, to compensate.
However, the value of 800 looks definitely too big, unless you are using a model other than the teapot, are you ?
If not, then I would suggest to try to play with the target size in the XML, trying different values, starting from the original sample size (247 width), and changing it by a factor 2 and see what scale you need to set...
I hope this helps.