To prevent disagreements over the guest list into awkward family disputes, the best approach is to ask both families for their guest lists very early in the process.
Say your future mother in-laws have legions of friends and relatives whom they insist on inviting - more than your own parents, in fact, and its putting you way over your number - yet they’re not offering any financial contributions to the wedding. If you have room for additional guests, have the groom approach his parents and ask them if they want to cover the costs of some or all of the guests that are putting you over the budget.
If that isn’t an option or you don’t feel comfortable with that scenario, the groom will simply have to explain that for budget reasons, they are going to need to trim their list.
If an increased financial contribution is not forthcoming, and they are still refusing to bend, you are in an uncommon and uncomfortable situation. If you have to tell them that their list must be cut drastically, try suggesting that you work with them to plan some kind of event, no matter how modest, in their hometown for their friends. In many cases, people just need to feel like they’ve included others in the festivities in some way, and a compromise like that can fit the bill.