JPEG -> PSD gives 10x size increase?

I looked around at a bunch of old threads about PSD size bloat and didn't really get a clear answer.

If I open a 2.6MB JPEG in Photoshop CC and save it as a PSD with no changes, I get a 26MB PSD.

Is this normal and expected? It seems a bit...much.

(Of course my application was a little more complicated, but not much. I opened a JPEG, masked two areas and applied adjustment layers. I wanted to save the PSD to save the workflow, but really the only information in it that mattered was the mask regions and the adjustment layer settings.)

Of course Photoshop should not be storing raster data with lossy compression in the general case, so it's easy to see why the file size is roughly the same as saving it as an lossless-compressed TIFF. But on the other hand, a lossless JPEG2000 of the same file is only 6 MB.

But I guess I would expect that a JPEG opened in PS with no changes to the background layer would preserve the data in JPEG format for file-size reasons,

at least until or unless the user did some transformation on that layer that would require changing the JPEG data.

So, what's the deal? And why is PSD compression so much worse than lossless JPEG2000 compression?

Thanks!

p.s.: I don't think it matters much, but the image is

JPEG 4032x3024 4032x3024+0+0 8-bit sRGB 2.381MB 0.000u 0:00.000

p.p.s: This is really a theoretical question. I don't actually need to reduce the file size, I just…want to.