Doesn't make any difference if you compress the images (this only reduces the app size on download & installed), art gets loaded into vram as raw textures.
C2 underestimates memory use in the editor because that assumes a few key factors which may not apply to mobiles.
The first thing is you need to figure out: is PhoneGap using the system WebView on iOS? If so, forget-about making complex games with lots of art assets and using PhoneGap. Just forget about it. iOS hates games that use above 100MB of memory and definitely anything 200MB+ is asking for repeated crashes. Do not forget that our testing devices are quite clean, user devices are bloated with many many apps running in the background.
You need to use iOS8+ improved WKWebView if you have a complex game.