General patterns:
Hens in general collect more fat that roosters generally do. Hens have a biological reason: storing up energy for when they are broody and sitting on eggs. But some hens really overdo it.
Young chicks tend not to have much fat, until they are starting to approach sexual maturity, and when they start putting on fat the females generally do more than the males.
Chickens that look slender and sleek and come from hot climates will generally not store up as much fat as chickens that look round and chunky and were bred to do well in cold climates. Very fluffy feathers can be misleading, with the chicken underneath being chunky or slender.
^None of those is accurate for all chickens, just general patterns that are true more often than they are false.
For specific observations: in a flock of mixed bantams, cockerel chicks with a Dark Cornish Bantam father had noticeable amounts of fat at butchering, while cockerel chicks with an Old English Game Bantam father did not have much if any fat. The cockerels were part of a breeding project that included tracking chicks by mother, and double-checking the father by comb type and other traits, so it was pretty clear that the father's breed was the factor that mattered in that case.
