The parents of the bride are paying for the wedding. They asked the groom's parents to make their list and not to exceed 50 people. If they wanted to invited more then the amount given they will need to pay for the additional guests. Is this appropriate?
Actually the couple sets the guest list these days as the couple is supposed to be responsible for all of the costs. But, if the bride's parents have set a budget and are allowing the groom's parents to invite, they have every right to say that they are not paying for any additional guests.
The parents shouldn't really be doing any of the inviting though--either set.