In order to make a lot of use of my webspace, I want to keep my displayed photos to a minimum of MB size.

Two photos of my page have exactly the same features:

Width: 700 pixels
Height: 527 pixels

Document size:
Width: 24.69 cm
Height: 18.69cm

Resolution: 72 dpi

However, one photo 76 KB heavy and the other 460 KB. Why is that?? How can I make the bigger photo smaller? I'm using photoshop. The quality of both photos is enough good for me.

Thanks for helpful answers!!