R.J.
What we all need in our life is more fear...no, wait a minute. That's not right.
Gack. So while it's PROBABLY just her own genetics have her being little...It's time to go to the doctor.
In my family about 3/4th of us are born with near adult sized intestines (slightly smaller, and then they don't get any bigger). The OTHER 1/4 have normal sized intestines that develop a "twist" in them. It typically takes about a year for the effects to be really noticeable...but it's the same thing you describe...baby is alert and happy...but getting thinner and thinner. The twisted intestines eventually twist shut. So while it had been difficult for the babies body BEFORE to absorb the nutrients it becomes a crisis, because now the food can't exit. Which in the old days would have equalled a ruptured intestine, and death, and now is a trip to the local children's hospital for surgery. Hurray for modern medicine!!!
Does your daughter have a twisted intestine? Probably not. Does she have one of the 6 or 7 things I can think of (from an autoimmune thing to worms)? Probably not.
The tests to find these things out are expensive...and since Every. Baby. Actually. Is. Different....it's pretty silly to run them all, if someone is just a little pixie by nature. That said. She's dropped significantly, and you've moved from concerned to worried. See your ped. If they blow you off, see another one.
If neither is concerned, take a deep breath and relax. You've probably just got a little pixie. AND people just HAVE to comment on babies. They'll stare and smile until they find something "special" about yours if something isn't immediately apparent. I think it may be a race-trait. Something that has survived since the dawn of time. Because no matter how annoying it it to new mum's you ALWAYS hear it, and once you get annoyed by it you catch yourself doing it! Big, tiny, strong, good lungs, looooong eyelashes, short little fingers, what a pretty smile....whatever it is. Most people are probably saying how tiny she is, because that's their "tag" for your wee one. Or because they've got toddlers (who go from teensy one second, to lumbering giants the next, as soon as their mum sees a real baby, and her heart catches in her throat).
R