The spritesheets on C3 are very large and are generated in a very random way.
Games that with C2 needed 200 megs of memory, on C3 need 700 or more.
This would be solved by being able to edit the sprite sheets, this is a problem for medium/large games because they need very large amounts of memory.
Construct automatically arranges spritesheets. This makes it pretty pointless to manually edit spritesheets: if you spent an hour carefully arranging everything, and then added a new object, Construct would automatically rearrange all the spritesheets to fit the new object, and overwrite all the manual changes you made. Instead we'd rather focus on ways to automatically improve the spritesheeting algorithm, or at least provide hints to it, as opposed to manually specifyng where images are located on a spritesheet.