Wrong model size on target

Heya we got a little problem in our project and are getting nowhere.

First our Project description:

We're developing an app in Unity where you scan a cloud marker and download the model with information provided by the metadata. This model is then imported into the app and displayed on the image target. Simple as that. The cloud recognition and download are working fine. We even got one model to work on its own target.

Now to the problem description:

We only got one model to work on it's own target. We're woring with the same scale for model to picture as these are real world buildings depicted. But the second model, using the same settings as the first, is displayed to big on it's marker. After a few different attempts at a solution I noticed that The second model is depicted with the right scale on the first marker and the first model is depicted to big on the second marker.

So of course I suspect some error with our marker but I am not sure where to find it and am pretty much at a loss here.