iPhone app photo format and size research

app with an empty imageView

app size: 37KB

app with a png in imageView

png size: 254KB

app size: 291KB

app with a jpg (quality = 100%) in imageView

jpg size: 131KB

app size: 168KB

app with a jpg (quality = 90%) in imageView

jpg size: 94KB

app size: 131KB

app with a jpg (quality = 80%) in imageView

jpg size: 66KB

app size: 102KB

Basically, app size equal to empty app size plus photo size. That means photo in app is NOT always save as png format?