I worked on some memory stuff for Figma, a similar app, though it's surely changed since I did.

One interesting thing about Photoshop's perspective is that it's fundamentally about "files", which is like a single bytestream that contains all the picture and which must be transferred to the browser to work on. A Figma document also can refer to images, but in a different way. The sum of the document data (like layout) and the image pixels can be extremely large, like gigabytes, but you can transfer those two things separately from server to client.