There are two very different reasons why people upload models to the 3D Warehouse.
One is to share a useful component so that others don’t have to draw it again. This is especially convenient for things such as windows, appliances, etc. from manufacturers because you can be sure to get the correct size and shape. These models should be well below 50MB if they are to be useful! However, many of them contain fine detail that is great if you need to view, say, a sink from 0.5m away but unnecessary in a model of a building. Always download a component into a separate file and check it over before adding it to your real model!
The other is to show off a modeler’s work. For example, if you just completed a model of an entire house you may want the world to see how wonderful it is. This kind of model is more likely to run up against the file size limit, either due to its sheer scope or because it contains too many components over-detailed components. It is not likely that someone else will need to reuse your entire 250MB model, so maybe it is more valuable to share images of especially nice aspects and maybe to extract individual components that would fit the first category. As Julia mentioned earlier, one might consider other sites and means to show off this kind of model.