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.