I don't have a problem with people wanting bigger butts as an option, but that gif, for example, the second option makes her butt just look -weird- and not at all fitting to the stance she's in.

She's standing with her shoulders back and her chest and hips pushed forward. Her ass would not be shaped the way you're asking when her overall frame is so petite, even if she had large breasts for her size.

It's still entirely human-shaped. Just because her butt is flatter than you prefer doesn't make it less human-shaped.

Really, what you're asking for isn't her butt being bigger, though. You're asking for her stance to be adjusted so she's not pushing her hips forward. Her hips should be pulled back a bit further, which would give the illusion of a larger butt (or rather, dispel the illusion of a flatter butt).

This assessment from the perspective of an actual girl, for what that's worth. To be honest, I'd be ecstatic if I had as good a body shape as the Miqo'te.