I'm not a designer, so please enlighten me. What kind of data can be held in a single image for a PSD to be 900 megabytes? Is it a single PSD for hundreds of web pages or?
*A PSD file has a max height and width of 30,000 pixels, and a length limit of 3 Gigabytes. Hence, Photoshop created PSB filetype which supports max height and width of 300,000 pixels and the length limit to around 4 Exabytes. https://en.wikipedia.org/wiki/Adobe_Photoshop#File_format
Depending on how hard the PSD format is to grok (purposefully or otherwise), this may be nigh impossible, but it also seems like it would be a great exchange format (which may be a another reason Adobe wouldn't want it). Then again, maybe there's already something like this...?