Thanks for the info about memory use. Very useful!
@Silverforce : I don't really know about what Phonegap Build is using. Do you mean XDK should be better? I will keep your tips in mind for my next game but here I need to find efficient ways to reduce memory usage on a finished game.
I have another new element to share : I'm using a "container" layout in my game that contains most graphical elemens of the game in it, and is never played.
The idea is that, as I have only one layout for every fight of the game, put all the enemies' images on the "game" layout will make it quite heavy. So I have them on an unused layout and only create the images I need at the beginning of each level. Is there any drawback to this method that I'm unaware of?
I just moved one of my images from my game layout to the container layout, and the "memory use" moved up from 298 Mb to 300 Mb, so I guess this memory use is for the "container" layout? But as this layout is never played, the memory usage displayed in the editor is probably not relevant, isn't it?
If I'm guessing right, it's the number inside the debugger which gives a clear indication of memory use (images only), which in the case of Healer Quest can go up to 120 Mb.
Another question : I have many big, invisible, flat-colored squares that I use as collision boxes in the game. Their weight is about 200 bytes each in my image folder. But do they use big part of the memory too?
Is partial transparency (i.e. 20% opacity) taking more memory than 0% or 100% transparency images?
At least I now see what I can do to reduce memory usage, so thanks a lot for all the very useful advices. And splitting my big layout into 2 parts (game+map) already seems to limit memory usage in game to around 90-100 Mb maximum.