I would like to point out, that GIMP made a huge progress in recent years :)

I am the author of https://www.Photopea.com, which is the image editor with the second best support for GIMP files (XCF) after the GIMP itself :) In the latest GIMP releases, I really appreciate Editable Text Layers and the support for DEFLATE data compression, which makes XCF files even smaller.

I created a document with a white background layer and a simple text layer both as PSD and XCF, and then ZIPped it:

XCF: 8.25 kB unzipped, 4.03 kB zipped

PSD: 81.10 kB unzipped, 7.96 kB zipped