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Personal Experiance:
Our cats have this exact harness. Luna has the emerald, and Gingi has the red, but they are both XS so I could switch their harness if I wanted to. Luna is about 6.5+lbs, and Gingi is about 8+lbs, it fits Gingi perfectly. Luna it is a tiny bit bigger on her but the straps are all adjustable so you can make them tighter or loser depending on your cats needs. We love this harness, it is a step up from what they had before which they were able to get loose in. This one they haven't gotten out of it once. 

Above is a photo of Luna from @GingiandLuna in the Purple Rabbitgoo in XS!

Below is Gingi in the light blue.