Photoshop sizing question

i’m working on the cover of a mixtape at the moment but seeing as it’s the first time i’m doing something like this, i got a little question

it’s a slim case cover, so it’s supposed to be height 240mm x width 120mm

the problem is,

  1. when i open a new 240x120mm file in photoshop, the imagesize is huge, doesn’t even fit the screen, eventhough the image measurements read 240x120mm
  2. when i have the psd open, the imagesize reads 240x120mm

but when i open the jpg in photoshop, the imagesize reads this

you can see the pixelwidth & height are the same, but the measurements arent

can someone tell me what i’m doing wrong? it isn’t supposed to be this huge, right?