Ok, so being from the girls perspective I will not jump at you for the fact that I disagree with you.
but, it depends on how overweight- if someone is obese- like AT LEAST size 16/18 or bigger it makes sense for others to tell. If someone is a size 10/12 (which I have no idea if that is what you are considering then there is nothing that they should say beforehand. You also have to ask the right questions if you want to get answers- i.e. what does she look like, etc.
Additionally, there are many shadchanim who believe that they don’t need to say something which a boy/girl will be able to see for themself on the first date. Why should a girl/boy not be given a chance at all because of being overweight. Someone can still be pretty and have all maalos.
Many ppl will not give it a chance lechatchila but once they see and get to know the girl, will reconsider.
the shadchan should know you and what you are looking for so that if weight is a priority for you she will know for next time.