Hmm, but that shouldn’t end up taking that much space either, no?
I can confirm that it indeed uses little RAM.
I was wrangling lots of big images and noticed that RAM usage was increasing steadily. Though that only seems to happen when generating new layers (e.g. by combinding them). Moving, for instance, does nothing. However I just noticed that doing things like adjusting the contrast (using the values histogram tool, sorry don’t know its English name) or adjusting the color temperature adds 300–400MBs. Do each operation three times on 9–12 images and you have a deadly amount of RAM usage.