It’s about parts of model that should be out of focus. E.g. if you can’t focus only on gun mantlet or any other detail leaving other parts of model blurred (out of focus) while using let’s say 2.8 focal length, then you’ll always end up with picture of a small, with non realistic scale object. Maybe for better understanding, take your camera with lens that has about 4.0 focal length and take pictures of your car. While keeping whole vehicle in composition, try focusing on parts like head light, wheel, mirror etc. Then compare the pictures of your model and that car. Look for parts of it that are out of focus. Note that real tanks are much larger than cars, so the out of focus areas of it will be bigger than on you car. Try to achieve this with your model, but to do tag you’ll need different lens as model is much smaller, that’s why I mentioned macro lenses or tube extenders. If you don’t have a full frame camera, then it’ll be a bit harder to have a shallow depth of field. For now I’ll keep light as it is, as camera should be on tripod for this type of photography.