The 3D viewer optimizes the model to fit it to a device's screen, whereas the AR viewer shows the model on a real-world scale. Therefore, it might look ok in 3D and too large in AR. To solve this issue, the model should be scaled down to the correct dimensions using 3D software.
Please note that the product's dimensions are defined at the modeling stage. If they are provided incorrectly, in order to have the product's real life-size, it might be needed to be remodeled/optimized at a real-world scale.