I think my sister is size 8, actually (6'...oh sorry """"" 5'11.5" """"", and she's been through two pregnancies). And I've certainly been thinner myself, even if I'm size 12 or 14 right now (and taller than 6'1"). And like technically I'm bordering on being overweight according to BMI charts (dipped into the overweight region briefly, but I'm back under). So...yeah, size 12 and 6'1" could be plus size in the sense of "technically overweight according to BMI charts".
Of course it doesn't LOOK plus size, which is the point.