This is the same issue as TIMODOPEN-228 but for Ti.Blob (core).
On iOS, all images created by Ti.Blob.imageAsResized are of type png (mime-type "image/png") instead of jpeg.
As a result resized images are much bigger (in terms of bytes) than what they're supposed to be had they been jpegs.
This seems to be related to the handling of alpha channel, according to TIMODOPEN-228
It works fine on android (Ti.Blob.imageAsResized returns jpegs).