I did the exact same thing when I got engaged. I started off with 9 b'maids, then cut it to 7 (but I actually WANT the 7 I have), with the 2 other ones, my best friends from secondary school, who I'm o longer that close to, I chickened out, I never mentioned it and just talked about how expensive everything was. I then invited them to the ceremony and wedding as guests. They weren't offended, at lest not openly, as the request had been a casual one, and I don't think they were actually expecting an invite to the ceremony etc., I think they were only expecting reception.
TBH, think about whether they'd ask you to be bridesmaids at their weddings? Would you be offended if they didn't? Just explain that its all too expensive and too big, they'd probably mind a lot more if you kept them in, didn't speak to them and made them pay for the dresses.
With regards to the ceremony/meal/evening, how would you feel if it was on the other foot? Is the cost of a few more meals worth more that their friendship to you? Not that you'd lose it, that not what I'm saying, but it is bit of a measure of a friendship.
If they are going to get snippy about it, really, they are old friends from school that you are not very close to and weddings are BLOODY expensive and stressful as it is. Call them rather than email or face to face, be apologetic but firm and clear, and then forget about it.