To be increasingly annoyed by a mum repeatedly commenting on my child's weight?

[MyUnicornMadeMeDoIt]

Just that really.
And how do I politely ask her to stop?

It has been going on for a while but more noticeable recently.
She says things like
'Well you need to stop having big babies'
~he is 14 months old and is wobbly when he walks, fell over and has a grazed nose~ 'Well with that much weight behind him'
~she goes to pick him up~ 'Cor you are heavy aren't you'
I wouldn't mind but he's actually my smallest child (out of 3) and (all) definitely not obese.

Next time when she says something:

'Yes he's lovely isn't he! bang on his centile- my aunt always makes funny comments about his weight just like you do, she's always struggled with weight so seems very fixated on it! You're clearly an ok weight tho, is there a reason you think you fix on it?'