Greyscale filesize increase?

[cm1194409]

So this is rather confusing.

I have massive images (16000px x 8000 px), in PSD and JPG, RGB8. When I try to save as greyscale to reduce filesize, I get a filesize INCREASE. What?

My 230 MB RGB8 PSD goes up to 620 MB in greyscale8, and my 2.5MB RGB8 JPG goes up to 4.3MB greyscale8 JPG.

Can anyone explain this?  Why would going from 3 channels to 1 increase the filesize?

[SteveWeinrebe]

You are right, it should be considerably smaller. Are you looking at the file size in your OS or at the bottom left of the Photoshop workspace under your document? Can you post a screenshot of the grayscale document open in Photoshop?

[c.pfaffenbichler]

Could you please post a screenshot taken at View > 100% with the pertinent Panels (Layers, Channels, Options Bar, …) visible?