12 week old baby's formula amount varies

[Tigger14]

We gave birth to our gorgeous baby girl at 37 weeks and she weighed 4lb 9oz

At 10 weeks she weighed 8lb 6oz...which is amazing. However the health visitor thinks she's not put enough weight on?

Shes formula fed. Her feeding amounts vary from 3oz to 5oz...does anyone's child food varies like this? Is it because she small?

It’s completely normal for them to vary how much milk they take. You will eat and drink more at some meals than others.

The important part is she following her centile line?

Completely normal to vary the amount of milk taken. My FF vary between 1.5oz - 4oz at that age. Even now he's coming up 6 months and vary between 5-7oz. I also think the weight gain is absolutely fine, but I'm not a medical professional and I don't know all the details so take this with a pinch of salt.
My FF 39 weekers weight gain was nowhere near that so that's my experience. What percentile line was she born on and where is she now? Usually the advice is that they can drop up to 2 as long as it then stabilises and tracks.
Can you ask for a second opinion or perhaps discuss with your GP.
Try not to get obsessed with the oz's - I breastfed my first and so I never knew how much he was taking and I never worried. Had to have a word with myself when I FF 2nd and was worrying about amounts. Look at the baby not the amount. Is she happy, content, alert, weeing and pooing plenty (6x wet or dirty nappies in 24hrs is a good guide.) does she wake for feeds?

If she's lethargic, sleepy, screaming with hunger, not wetting nappies etc then sure worry but otherwise I'd presume she was well & get a 2nd opinion regarding her weight and go from there.