Why is the canvas size so large for such a small image?

After removing many layers and details from an existing photoshop file, I only have one more image left in it. However, the canvas is very large and the image is very small. This is frustrating because when I open the image as a JPEG in preview (on my mac), the image has a lot of white space in the background.

How can I get rid of this white transparency background? My image is not easily cropped- it is a curved image. I can't upload a picture of my situation but it's basically a large checkered canvas with one layer- the image layer. The one object on this canvas is a small curved rectangular box with some text in it. Again I cannot crop this with the crop tool because some white is left in the corners as a result.

Any help will be appreciated. Thanks.

cant you just select the area>copy it> and create new file size you need, then paste it to it and use the transform tool to size it to what you need. Then save it as a single image and drag it to where you need it?