Lol... extremes? 10 lbs is extreme? Thanks for making me feel like s***... 😛 (I'm like 30 lbs over my "ideal" weight.)
In all seriousness, though, it's not so much what you weigh, it's more where that weight is. In my opinion I'll always take someone who's a little bit overweight over someone who's a little bit underweight because I love people who are softer and curvier over people who are bony and frail. But again, it depends. Gendered fat deposits make up a HUGE part of how we differentiate the sexes. So to my eyes, early in transition super-thin MtFs are a bit more passable, because their baseline is more androgynous, and their lack of gendered fat means that usually they can start being gendered female with simple cues such as long hair. But later in transition, MtFs with more fat definitely gain the upper hand. Because once the body starts putting on "female" fat, it can really cover up a lot of the deficiencies in bone structure, where the thin MtFs will always have their male-ish bone structure more visible. If someone has boobs, a butt, round cheeks, and female thighs, it becomes WAY more difficult to clock them, even if they have some bone-structure deficiencies.
Most of the completely-passable trans women that I've met are in the "a touch overweight but still slim" range, with good curves. Again, those curves can make a world of difference.