which looks like the camera is just zoomed into the island rather than that the island is bigger,
... but, but that's exactly what is happening when the island is bigger. The pixels get bigger.
how can I maintain the size of the outline pixels no matter the scale of the island?
By not using pixel art.
For continuous zoom, pixel art does not work. For fixed zooms you use different pixel arts. Or just zoom in regardless, so you just use a highest resolution pixel art and zoom in, instead of a low resolution and zooming out.
Maybe a vector graphic can achieve this. Or some graphics filter.