trimmer engine displacement question
I am looking at new string trimmers. I noticed (and asked the company) that Echo does not give hp ratings because they say there is no standard to measure it. They say engine displacement is key. That being said, if you look at a Stihl KM90R's displacement, it is equivalent to the Echo PAS-280 (or the lowest commercial Stihl to the highest commercial Echo). I'm not sure I buy that the "best" Echo has is the same as the "least" Stihl has. Something has to equalize that.
So, to the question: how does engine displacement matter? How can I compare different manufacturers - straight number for number?