Well, at least she was open with you.
I'd much rather get 'you're too fat,' than 'Um, I really think you'll be uncomfortable because... um, there will be lots and lots of people you won't know, and I sure don't want you sitting there not knowing anybody,' or 'Gosh, I'm SO sorry, but my fiance's mother invited the entire starting line-up of the Philadelphia Flyers, and I know how much you hate hockey.'
I've gotten similar veiled messages in the past, and stupidly, I didn't read between the lines. I'd be all 'well, I can come earlier, or wear something different, or not bring my boyfriend, or... ' I'd have much rather the person just came out with what she wanted to say, i.e. 'I don't want you there.'