my now 11 y old was that weight at 5-6 months
he was practically 11 lbs born and 29 inches(yes 29 inches and no im not making it up as ive been accused of that before on here, he really came out at 29 inches with enormous feet) though and in 6-9 month clothing from new-born where most new-borns are on a few ounces a bottle mine was a full 11oz then at 5-6 weeks 16 oz every hour and half. he used to go through 2-3 tubs of formula per week.
i got that fed up he went of full cows milk at 6 months.
,hes been off the charts since birth as he was always well over the 100% percentile
i never had a hv anyway or took him to any clinic to get weighed (all i did was injections at the doctors)so it never mattered as he was never weighed and measured for their charts, I used to do it myself every month.
hes 5ft 5 now and just over 11 st in medium to large mens clothing with size mens 9 feet.
loads think hes 14/15 but hes still technically a primary school kid in y 6(we home educate so thats why i said technically)
hes been way way over his peers size(and a few years older)since birth, ive never bothered worrying as he wasn't standard born.
some people have 4-5 pound tiddlers that fit in one hand some have 10-12 pound whoppers that need 2 hands to hold a newborn.