Hey Sarah
Unfortunately, wedding guest lists are almost always a sore spot. You know, I wish someone would air a wedding ettiquette program for brides and grooms AS WELL AS guests. I can't stand it when as soon as someone knows that I'm engaged, that they say stuff like "I'm waiting for the invitation!" So rude and assuming

--how do you know I'm not just doing something very intimate with just our immediate families?? I have NEVER said that to someone, not even to a close friend or relative, because I don't know what their budget is, what their situation is, maybe they want to elope, maybe they only want their parents there, maybe their just doing a civil ceremony with no wedding festivities, etc.
You just have to stick to your guns with your guest list and figure out what your priorities are. If it's important to you to have your whole family there (these extended relatives you're talking about), then just cut back elsewhere on some things, like decor or choose less expensive food options. If what's most important to you is having certain lavish touches that must be there, then you're gonna have to be brutal with the guest list.
Or maybe if there are extended relatives that you have to invite, you could just invite one particular generation. That's what my DF and I are doing in many cases. Because otherwise, it would become so and so and their kids, and their grandkids, and then it's like, 'well, where does it end?' We would easily be at twice our guest count! I'm also doing that for many friends of the family as well as my extended family--many aunts and uncles will get invites, but not their kids/my cousins, unless they are young children or babies who can't stay home alone when their parents go out of town. Fortunately, the majority of them are much older so this is seldom the case. But if their kids are in their 20s or 30s, I'm not inviting them, unless they are extremely close to us, as in we hang out a lot, we spend a lot of time together. If it's a cousin I'm not close to at all and that I see every 6 years or so, I'm not inviting them. Sounds brutal, but with a guest count of 300, I have to be because it's important to me to get what WE want for our wedding too.
If anyone says anything, you can simply, in a nice manner say that you had to stick to a certain number for the hall you chose and you're already over that number (it's not like it's really a lie!

)
Be aware that it's common to make enemies through your wedding by leaving certain people off the list. You just have to be alright with that and tell yourself that if someone can't understand your budget concerns or your wishes for an intimate affair, they're just being petty and selfish.
So good luck, and keep us posted!