interedasting.
here's what i noticed first - you mentioned that it's all a big image of 100mb. okay. so i presume that layout has that one big image + a lot of other content.
problem could be that your whole layout is loaded, assets are loading in memory more and more, but noone of those which are not shown on screen are not removed from memory.
also, try updating your node webkit and construct. also try setting behaviours "destroy outside layout" for all the sprites / stuff, even when changing layouts the last layout stays loaded as it was.
also have you changed downscaling to medium (high is no more different then medium but uses almost double memory). also check the tips here (i know you said you did, but double check for any case - https://www.scirra.com/manual/134/performance-tips and https://www.scirra.com/manual/183/memory-usage)
another problem could be music / sounds that are wrongly set. they could load infinitely times into memory if not used correctly - therefore higher usage.
i don't know what else i could give you, but i hope you find your problem and share it with the world! <img src="{SMILIES_PATH}/icon_e_smile.gif" alt=":)" title="Smile">