After reading this topic I opened a RAW file from my D300. Can't get the exact files size from my MAC other than to know it is less than 100MB. I used NIK Silver Effects on a duplicate layer generated in Affinity and saved the resulting file in PSD format by using the export option under FILE. The option calculates file size before saving and this one layer RAW file with one layer calculated to be 204MB when saved in PSD format.
I then opened the PSD file in Photoshop Elements 12 which had to convert the bit depth as it will not handle anything over 8 bits ( I'm sure this saves space). The file opened without a problem and looked great. I then saved the converted file in Photoshop Element PSD format and looked at the resulting file size. It was less than 100MB or a fifty percent reduction (again bit depth no doubt came into play here), but the original RAW file was also listed as less than 100MB. Is Affinity saving at a greater bit depth in PSD than native Photoshop? I don't usually get this far under the hood on these, but considering how fast I chew up disk space with my shooting I thought it a notable topic.
I should note that the EXPORT option will also calculate file size for PNG,JPEG and TIF formats, all of which are significantly lower than PSD.
All that said I would never save to PSD format unless I had a client who wanted files in that format. I would most likely save for archive as a TIF and JPEG for web use, but Affinity does seem to expand the file size when exporting layered RAW images in the PSD format.