> Peregrine falcons adapted quite well, and they're much more sizeable.
I'm not sure what you mean. As far as I am aware -- and according to every source I've looked at in the last few minutes -- Peregrine falcons and Cooper's hawks are about the same size (length and wingspan are within 1-2 inches).
Peregrines are somewhat smaller and much more lightly built. I live in a nesting pair's territory which often sees transitory Cooper's; they're easy to distinguish both in flight and at rest. Male Cooper's are more peregrine-sized and hard to tell from sharp-shinned hawks sometimes, but that is an ordinary enough sexual dimorphism in birds.
Interestingly, while peregrines and accipiters like Cooper's share a habit of taking passerines in flight, the response of potential prey seems to differ. I frequently see songbirds mob a Cooper's; I can't think offhand of a time I've seen them respond to a peregrine other than by crypsis.
Sadly we don't have Cooper's around here so I have no experience with them, hence why I looked them up (see nearby comment) and according to that source found out they were on the smaller size and much smaller weight.
Around here the only ones who would dare mob a peregrine would be crows.
Sure. I'm just talking about the impression they give in life. But I suppose in that sense the other birds must find a peregrine much more striking, and it very belatedly occurs to me that peregrines no doubt look smaller and more gracile to me for the higher altitudes their stooping hunting habit would require. When I do occasionally see them on approach to their nearby nest, I'm struck by their relative size. So yeah, between that and reviewing my Sibley's the error here is mine, though - for that matter, likely also because - I do find all falcons rather streamlined and compact in impression compared with accipiters or buteos.
> according to every source I've looked at in the last few minutes
I may be mistaken but that's what I found:
https://en.wikipedia.org/wiki/Peregrine_falcon
> The peregrine falcon has a body length of 34 to 58 cm (13–23 in) and a wingspan from 74 to 120 cm (29–47 in)
> Males weigh 330 to 1,000 g (12–35 oz) and the noticeably larger females weigh 700 to 1,500 g (25–53 oz)
https://en.wikipedia.org/wiki/Cooper%27s_hawk
> Total length of full-grown birds can vary from 35 to 46 cm (14 to 18 in) in males and 42 to 50 cm (17 to 20 in) in females. Wingspan may range from 62 to 99 cm (24 to 39 in), with an average of around 84 cm (33 in)
> In northern Florida, males averaged 288 g (10.2 oz) and females averaged 523 g (1.153 lb). In general, males may weigh anywhere from 215 to 390 g (7.6 to 13.8 oz) and females anywhere from 305.8 to 701 g (0.674 to 1.545 lb), the lightest hawks generally being juveniles recorded from the Goshutes of Nevada, the heaviest known being adults from Wisconsin
(not putting the full regional rundown, just the biggest entry)