If your MCHN is not worried then I probably wouln't be worried either. Has she been gaining weight fairly steadily? Is she happy? Is she learning to do new things ie: rolling, able to sit up, maybe crawling? Are you a small framed person, is your DH small as well? If the answer to these is yes then I wouldn't be worried.
If you're answering no then I would be a bit worried. In that case it can't hurt to get a second opinion.
One of the mums in my MG has a son who sounds similar. She is getting no end of hassles from health professionals. There is nothing wrong with him except that he is a small baby and does not fit onto any of the 'normal' charts.
HTH