Image Size Problem

[chrisjo 0]

I am new to this product and have a question about file size. I created an image with a file size of 640/960. After I got it just where I wanted it and saved it. However, when I reopen the image in preview the file size is 300/450. I need this image to be 640/960 pixels. Can anyone help me out? Why is it decreasing the file size? Thanks

[KD-did 140]

I formatted my canvas by unchecking the top box and never had the issue with resizing.It stays at the set size I entered,