Here is a quote from Ashley regarding image sizes:
[quote:1xz36cf7]Some people struggle with memory usage on mobile devices with Construct 2, but usually from lack of awareness to how images are stored in memory. Your game might be a 5mb download since all the images are compressed as PNG or JPEG, but they are extracted to uncompressed (basically BMP) in memory for rendering, which can easily make it 5-10x bigger in memory than the download size. Because lots of people ran in to this we recently added a memory usage tracker in the Construct 2 status bar. It's just an estimate, but will give you a useful approximate figure (e.g. 5mb download size, 40mb memory use).
Source is Here.
More useful info can be found in Ashley's latest blog:
Understanding CPU and memory measurements.