Hi M.,
I'm your extreme and I don't want to alarm you as I'm probably the exception to the normal where kids just grow at their pace. My doctor always said: "babies can't read books so they don't know what averages they should aim for."
My daughter was similiar to yours, normal birthweight, dropped weight first week, slow, slow gain, not big eater, i was worried about her from Day 3 of her life and her eating. (She's my 3rd child so I had some comparisons.) At 6 months she had a few incidents where she did larges spits/vomits. Seemed like just a tummy bug to me but as I still worried about her so took her to pediatrician. He listened and this time decided there was more to what met the eye: and suggested she may have reflux - mostly 'silent reflux' (i.e. where she swallows it again so doesn't do much external vomitting). We saw the specialist and yes, she did have reflux. We started giving her medicine for reflux but she'd just vomit it up mostly - she has a hair trigger gag relex. She still did poorly because babies are smart enough to figure out whenever I eat I get this uncomfortable reflux sensation - so she didn't want to eat much. And she ate primarily formula (as it has more calories then any other food) and some youghurt.) We took the 'wait and see if she'll grow out if' approach. Finally, at one year she got another tummy bug, lost weight and pediatrican and us decided she wasn't going to out grow - or even survive this - this without more invasive help: So in to hospital we went. She had tests run to ensure the right diagnoses and when it all came back negative, with just severe reflux, she got a gastrointenstinal-tube (g-tube) put in her stomach so we could feed her thru that - and really control the food intake - and finally give her medicine that way too. (It was great. You can drip fed her while she's sleeping. I never realized how stressed I was about her eating until I could truly control it.)
To keep the story short: She is 25 months now. She's always been the happiest, smiliest baby - even with all her trails and hospital visits. She still has reflux and reflux-vomits unpredictable about once a week but she is 15 percentile now, trying hard to keep up with her 2 older brothers (but is about 6 months behind developmentally probably because she was nutrinally lacking in her early months), is learning how to eat 'normally' i.e. orally and we hope to begin to wean her off the g-tube this fall.
I'd expect your daughter turns out be quite normal and not as extreme as mine. But for ease of mind I do suggest you check with your pediatrician if reflux could be a consideration.