Doesn't sound to me like they went behind the grooms parents back. It sounds to me like the grooms parents don't care about the arrangements just the number of people they can invite.

I didn't say behind the grooms parents backs , I said behind their daughters back

Why did the brides parents have to involve the bride in them meeting the grooms parents at the hall so they could show them that the hall was too small to accommodate more people?

Perhaps this is a case of groom's parents wanting a big wedding while the bride and groom do not. It's 4 against 1 (or 2, depending on what the father of the groom thinks) here. Don't let them bully your daughter and son-in-law into something they don't want on THEIR wedding day. The parents of the grooom could invite as many people as they wanted to THEIR wedding, why are they trying to make their son's wedding theirs? Bully back, and don't give in. This is ridiculous. Good luck.

To avoid what happened afterwards... the grooms mother crying to the bride and the bride being upset by the fuss

Ok, I think this argument is serving no purpose. I think the OP has gotten advice from different viewpoints, So lets not argue further about what the OP should have done. That's in the past. If anyone has any further insight on what they can do to prevent violating the law by inviting more people than the hall can accommodate, please contribute.

Scott is right. Also the op only posted 1 time and has not been on since 9:03 that day.
Maybe she was just venting and all is settled now.

Sorry I have not been on since I posted. Things have gone from bad to worse. Anyway, let me explain exactly why we talked to the groom's parents without telling them. It was not about paying for the wedding and wanting control. Our daughter said from the day she got engaged that she wanted round tables. When we went the to florist, the centerpieces that she picked out were for round tables. She picked out the fabric for the runners for round tables, which I have already made. She has pictures of reception halls with round tables earmarked in bridal magazines. The simple fact of the matter is that she wants round tables and she will not have round tables if the groom's parents do not cut their list. She is a people pleaser and wants her future in-laws to like her and as she has said "not be mean to her". Her fiancée is a huge momma's boy who at 25 just moved out of his parents' house. He knows she wants round tables, but will not talk to his parents about cutting their list. Instead he forbid us to talk to them about the their list. That is why we did not tell our daughter or him that we were meeting with his parents. It has never been about the money. It is strictly about the number of people the hall will hold with round tables. The reason I think the groom's parents do not want to cut their list is that the mother of the groom has already verbally invited everyone on it. I emailed our daughter and suggested mediation to solve this situation. Has anyone ever went through mediation? Any tips?

Mediation by whom?

If the groom's parents have already invited people, that's their problem. They can go back to these people and tell them, that the hall you decided on can't accommodate as many people has they thought.

Our younget daughter suggested we hire a wedding planner or wedding coordinator. They are knowledgeable on wedding etiquette. All parties would have to agree to abide by the mediators decision. I don't think they will agree to it because they have not been willing to cooperate in any way yet and I think they know they will be told to cut their list. I think they know they are being unreasonable, that is why they have been having the groom run interference for them. He continually looks out for his momma's wishes and not our daughter's. Our daughter is spending the weekend with her future in-laws at their cottage. We have not talked to her since she called last Sunday. They have shown over and over again, they will say or do anything to have their way. Our daughter and her fiancée are going to stop on their way back from his parent's cottage to talk. How do you suggest we handle the talk?

The only thing I can think of is getting another hall nearby for the over flow even though the bridal party would not be at that one at least they would have something more than N0 invite.
If not a hall, as I suggested before a wedding tent at a park or someone's house who would agree to it.
Other than that if you 'have to' invite these extras, the only thing I can see happening is canceling the hall and get another bigger one. The wedding coordinator wouldn't be able to fit more people and they would most probably not come up with any different solutions other than the decision now being on them instead of you.