the compression of the file and the memory it take are just 2 different thing at all.
you could use bmp file if it doesn't matter.
file size = memory size.
I know that C2 allready have some optmisation on png the most major one is the "atlas" way it use to eleminate duplicated sprite from the same source.
OptiPNG just optimise the png file without loose quality and don't change decompression time significatly (and after all the memory take is the same).
BUT in mobile, where the actual solution Cocoonjs or IndelXDK take a big part of the project place (xx MO), THE SIZE IS REALLY IMPORTANT.
You can MISS players just because the game is too heavy to load with a slow connection.
And it's exactly the same problem with HTML5 game.
More the data will be small, less the loading date will be (downloading the data) the most important point for mobile+html5 plateform.
For the people using badly Construct2 that an other problem, tell all programmer that come from classic C/C++ background on any SDK from any mobile plateform and they will not have any problem with C2.
C2 just give people without any technic or programmer skill the ability to make visually code.
But they just have to learn and understant some limitation for the target hardware, the blog post : scirra.com/blog/112/remember-not-to-waste-your-memory
what done in that way, for people without programmer skill to learn that all hardware as limit :)
But we can't not use a tool for make some optimisation because of fear that some people will make bad use if it (Just Uncheck this option by default :))