No they shouldn't Clare... u/s measurement of the baby prior to 12 weeks gives the most accurate indicator of gestational age. This is because embryos grow at a fairly consistent rate. After 12 - 13 weeks a couple of weeks into the fetal stage - familial traits kick in - babies can be a bit smaller than dates or as in your case bigger and this is just their genetic make up.

My babies often measure ahead too - and I have larger babies... *Bob* is a healthy little baby growing beautifully!