Weddingbee boards

To make a long story short:  My fiance and I have selected an inexpensive, outdoor wedding venue which we really like.  Its maximum seating capacity is 170, which means each family gets to fill 85 seats.  This is already larger than what we'd prefer, which would be around 100 guests total.

I'm an only child, and my mom has gotten it in her head that we must invite allllllllll of our very large extended family, including -- I kid you not -- third cousins, relatives I literally haven't seen since I was 13, and a few distant family members I really dislike.  In addition, she wants to invite high school friends of hers I haven't seen since I was a toddler.  (I'm 23 now, and have been financially independent and moved out for over two years.)

My mom is paying for most of the wedding, and I have already made a number of compromises on both this and other issues.  She will make no compromises, and even went through a brief phase of -- whenever we disagreed -- telling me out of nowhere that she thought the dress I'd picked out was ugly.

But back to the matter at hand:  We're on a tight budget, but she's horrible with money and doesn't seem to understand 1. that we can't afford more than 170 guests and 2. that the venue can't seat more than that. 

In fact, in her quest to ensure she gets to invite whomever she pleases, she has actually suggested that if all else fails, she will send out her own rogue invitations, hire a second caterer herself, and rent a tent + additional seating, which she will put up herself on the lawn of the venue for the extra people.  As outrageous as that sounds, I honestly wouldn't put it past her to do it.

Needless to say, the whole rogue invitation plan isn't going to fly, and this ongoing guest list fight can't continue.  I'm brand new here, but I'm really hoping you all have some suggestions for how to move past this successfully.  I've already tried the A-list, B-list, C-list thing, which only resulted in her saying that I don't care about my family and dramatically suggesting that I just elope.  Thanks in advance for your help :)

@creativeplannertobee: Totally agreed.  But all the say?  Here's a small sampling of the say she's already gotten:

• I got a new dress because she didn't like the vintage dress I'd altered because it was "too casual and she couldn't underdress me if I wore that."
• The menu has changed from a simple ice cream social to heavy hors d'oeuvres.
• We're having a cake because she wants one, even though I don't.
• Most relevant here:  The guest list has already grown from 100 to 170.  We'd have to change venues, losing our deposit and paying thousands more, to accomodate the list she now wants.

As I said in the original post, I'm trying to compromise and include her on as much as possible.  But there's literally no room for additional guests -- and she won't budge an inch.

(I should also note that it was originally my plan to pay for most of the wedding, but she won't allow it.)

UPDATE:  No, she knows the numbers.  She's barely able to pay for what we have planned now, won't let me pay for it, and still has this plan to invite more people without apparent regard for the additional cost.

@mrsjjohnson2b: Right now only 30% or 40% of the guest list would be people I'd invite.  The other 60% to 70% are people she wants. 

And she wants to add another 30 people on top of that who would be 100% people she wants.

The college analogy is really good, though -- maybe I can bring that up to her :)

@Peony007: Thanks :)