Who runs these ante-natal classes? It sounds incredibly unprofessional.
I really think you should consider complaining, especially if its NHS classes.
Ante-natal classes are supposed to help you and support you. Individually picking you out and telling you things like that is NOT helpful in the slightest, and as a HV she IS NOT QUALIFIED to make judgements like that. At most what she should have said are general comments about larger babies, rather than singling you out. What she has done is effectively 'diagnose' you as having a large baby. Just over 8lbs whilst over the UK average it is NOT even considered to be large medically. The NHS website says that 8lb 8 is considered to be the lower limit of macrosomia - and a lot of consultants consider it higher. 8lbs is just in the upper ranges of average.
And yes, your second baby is generally bigger. If you had a baby that was 7lb 12 previously then your body is designed and has proven ability to cope with a baby of that size. 4oz extra does NOT make all the difference. I would love to see the scientific evidence that says this, as everything I have read says entirely the fucking opposite - it suggests its pretty bloody normal. Anecdotal stuff I've read seems to suggest that slightly larger babies can almost be easier to give birth too, as well.
In terms of a growth scan, I would wonder what this is going to achieve for you? Are you measuring significantly beyond your weeks or is the baby's growth more than was predicted for you? Are you diabetic? Are you overweight? (Pretty sure you have said you are neither of the later two). Have you any other reason to worry or be concerned about the size of your baby based on anything other than the opinion of this woman?
If you have, then yes, maybe there is a reason for a scan. Otherwise, a growth scan could actually just fuel your fears - it is liable to say that the baby is health, just a bit bigger - as after all you are not getting any other symptoms or problems.
If they were just to come back and say it was going to be 8lb or slightly bigger, its just going to freak you out now. Unless it was significantly bigger than that, they won't even suggest early induction or a c-section. And would you want either of those anyway? Does this leave you in any better position than you currently are?
What you need is reassurance, that even if this baby is over 8lbs, that you can cope with it, and thats what I think you should ask the midwife rather than asking for a growth scan imho.