both my DC were biggies at birth - 8lbs 7oz or thereabouts. a few months on, they were in 3-5th centile and stayed that way for years - i was a 11lb baby at birth and looked like a balloon in baby photos. My babies, short/very lean and visibly less chunky/shorter than other babies same age.
DC2 - 8 has become more average now but still is quite small in terms of height/weight - i'd guess 60% of her class are bigger than her.
DC1 is still tiny - i was actually a bit worried about her height and weight - still 3rd centile, very tiny and lean, girls in her year are broader and head/shoulders taller than her. When I was 12, i was 5 ft 5 towering over 99% the boys and girls in my year!
I'm 5 ft 9, so not huge but taller than most women i encounter.
DH is 6 ft 2, again, tall but not unusually tall.
If the weight has rapidly come off and there are only minimal gains at a very slow rate and other unusual or atypical factors, maybe speak to HV?
13 month is still very young and 50th centile is very respectable.