When the groom wants to invite way to many people

He JUST graduated from marine training? He may be deployed randomly.

How do you know that your daughter and future son-in-law can't pay for their own wedding?

Tell daughter and FSIL how much you will contribute to the wedding. Be sure you can contribute that amount - some parents of brides on here have backed out of what they promised. Let daughter and son in law plan the wedding and figure out their budget. They are adults, they can do this. If they have the budget for a big wedding, fine. If they realize they can only afford a small wedding, fine. Either way, they need to figure this out - not you.

Tell him to figure out a budget first- money he has in hand for the wedding- and do some research on what can actually be bought for that amount. It will probably be a reality check for him and he'll cut the guest list.

Offer to help with a certain thing and/or a specific amount. That's it. Let them plan their wedding. Honestly, if you say something to him, you may come across as the nasty mother in law.

Have they sat down and wrote out a guest list yet ? Did they have a budget ? Maybe seeing it in writing will make him see that the budget and guest count don't add up ?

You can only ever have the wedding you can afford. This is true for everyone.

It's very, very generous of you to help out financially. Tell them how much you're willing to contribute and then let them figure out how much they can afford and let them create the budget. From there they'll figure out how many people they can properly host. (Note that proper hosting includes a meal for a mealtime wedding and wine and beer at a minimum. Heavy apps or hors devours and wine and beer at a non meal time)

I agree with the previous posters! Either giver your daughter an exact amount that you will be contributing and they can plan however they want, or you could offer to pay for specific things (like the dress and bridal session)

Write them a check that your'e comfortable with and bow out. Whisper under your breath, "get over it", and be done with it.

He needs to come into this decade, where wedding cost a shit ton, and the bride's parents are not expected to foot the bill.

I agree with the other posters, you let them know what you can cover or how much - then they're adults and need to figure it out. Alternatives like Friday or Sunday weddings or a DW may be a good option for them.

Agreed with others. Contribute what you can afford and let them deal with the rest.

Allow them to piece out how much the things they want cost, including venue, food, clothing, music, photos, flowers, rental elephant, rings, and whatever else, and then show them how much let's say, $1000 of you contribution, will go.

My coworker's daughter wanted bigger and bigger flowers at her destination wedding last month. My coworker had to show her that for this upcharge in flowers, she could furnish three rooms in her new house that had no furniture. She chose the flowers.

I also agree. Contribute what you can. That is very nice of you to help out. Give them the check. Let them use it how they please, and let them do the planning. If your daughter is old enough to get married, she is old enough to figure out the guest list with her fiancé. They can compromise with each other.