The challenge for the shadchan is trying to gauge what a person finds “attractive”. Even if the shadchan personally considers a client overweight, it doesn’t mean that the person going out with this client will agree with that assessment.
Of course, as others have said, morbidly obese may be a whole different issue. The shadchan would most likely notify the obese client that this information will be revealed beforehand to prospective dates. It’s then up to that client whether to proceed under those conditions.