Good day!
jpg is a lossy compression method (which does not mean that it’s bad per se, but one should ideally only employ it in the last step, the creation of the ultimate file for print or web), psd files are compressed but not lossy so the ultimate size is likely to be larger.
Also, depending on the Preferences and settings, psd files contain the Layers and a flattened preview, so that can in some cases increase the size significantly.
Regards,
Pfaffenbichler