My mom taught early-ed her whole career. She just retired after 32 years of teaching, the majority of which was in kindergarten and 2nd grade.. She had a special early-ed certificate on her diploma. Her Masters degree was in early-ed and she studied the effects of starting kids early. If anyone knows what she's talking about with your issue, it's her. Here is what she would tell you:
Size almost never matters, social readiness does. If your daughter is already shy, you'll make it worse.
You may not see a big difference now, but the difference between a 2nd and a 3rd grader, a 5th and a 6th grader, an 8th and a 9th grader, and a 15- and a 16-year-old is immeasurable. It doesn't matter if they have grown up together, aging takes place no matter what the social setting is, and while her friends will be growing out of things, she won't be ready.
She may do just fine if you put her in early. But she could have been AMAZING if you left her back.
An early readiness for kindergarten is no proof that they will stay ahead. There's actually a lot of research that shows that it doesn't matter if you start the child early, they won't do any better later on than they would have if you had held them back.
Lastly, you can always skip a grade later on if she really turns out to be "gifted". It's really hard to test for a true IQ at her age, so anything you've done to get that score doesn't mean too much. If she gets to 2nd or 3rd grade and is so far ahead that she's bored, you can skip her and she'll get an ego boost. But, if it turns out you are wrong and she struggles for B's and C's, or worse, has to be held back, it will be your fault when she suffers from self-esteem issues, and I promise you, she will suffer.
No one will care when she is applying for college if she's a year ahead of schedule, especially if she's getting solid B's. But if you keep her back, let her be in accelerated programs, and get all A's, colleges will be jumping at the chance to get her.
Good luck with your decision. Talk to the schools and make sure you listen to their suggestions. You are looking at one child compared to a few others you've seen. The school and the teachers have seen THOUSANDS, and in this case, know what's best for your child better than you do (unless I missed the fact that you are an early-ed teacher, which obviously would have some sway).