my dd was 10lb 2oz at birth and weighed 15lb 4 at 6 months so i wouldn;t worry. dd is thriving now and even at 20 months shes still only 18lb.
as long as your dd feeds and is thriving, is happy and contended i wouldn;t worry too much. the health visitor just goes byt the centile charts, buty i think a smy dd proves sometimes they come out bigger and aren;t emant to stay big.
ds1 was 10lb 4 at birth and weighed 18lb at 6 months, and is now the skinniets lankiets thing there is, he eats for england too, form the moment he gets up, comes home form shcol hes alwasy eating but hes so active he burns it all off.
whereas ds2 was 10lb 4 oz at birth and is still a chunk, he wremained on the same centile for height and weight but has had a growth spurt and is in 7-8 clothes for the height now, but he still has a little rounded belly, hes 5.