Disagreements over the size of the wedding? Ultimately, the choice lies with the bride and groom, within the constraints of the budget. Now, that isn't to say that the couple ought to veto whomever they want from the wedding when the parents are helping to foot the bill. Make exceptions where they make sense. If you set the limit that the groom's family at 50, and they want to invite 52, you should be reasonable. However, if you set the limit at 50, and they come back with a non-negotiable 100, that's another story.
Explain to both families the size of the wedding you want, and ask them to work within that list. In some situations, it may help the families to understand why you want that size wedding. Perhaps you have a limited budget, and would prefer small and elegant, to large and basic. Or maybe you just see a wedding event as an intimate gathering, rather than a massive party. Send wedding announcements, following the wedding, to the acquaintances who couldn't be invited.