I wonder if perhaps the confusion comes from false conditioning? What people seem not to realise, it is not so much the centimetre measurements that matter here. Who can honestly judge accurately a 10 cm difference in wingspan of a bird soaring 300ft above them? - of course not. However, it is the size of the body (exc tail) and broadness of the arm/wing base and hips are where the Goshawk gets it’s real size difference, even, very noticeably, between a male Goshawk and female Sparrowhawk.
People, imo, fall into the trap of thinking there’s little size difference between the latter two because of the field guide method of describing size (ie by wingspan and body length) .Birdwatchers perhaps then, in some small part, fall victim to auto-suggestion in that it is difficult to separate these species in the field. In other words, Field Guides have conditioned people to the idea that body length (inc tail) and wingspan (wing length) are the same as body size (ie bulk of body + tail) and wing size (ie wingspan + breadth). Goshawk -v- Sparrowhawk demonstrate that in the field, clearly this is not the the case. Of course posture/flight behaviour, time of year, even time of day, can impact on size perception of birds in the field, which means studying the bird for as long as possible.
In my own experience, I find that birds soaring on open wings that have a larger body mass, with a proportionately shorter tail, (so similar in length in cm), and broader wings (with a similar wingspan in cm) appear much larger (and are actually much larger in surface area) than other birds that have a similar wingspan and body (inc.tail) length but instead have a combination of a smaller body+longer tail and narrower wings.
Cm size comparisons depend on proximity to be able to assess size difference, the narrower the margins, the nearer the bird has to be for that to be helpful field guidance imo. At 800ft for example, 10cm is largely irrelevant!
