Changing Image Size for Reader

Hello. I have a question about the images that display in the reader. Whenever I make a post, the image in my reader is kind of small, and they don’t fit the space well, leaving a lot of white space to the left and right of the image. However, many of the other posts in my reader made by bloggers I follow have very large images, and their post takes up more vertical space. I want to make my images as big as theirs. My Images are generally 1500 by 2100 on my computer, but when I download them they have been reduced to about 730 by 1024. I am using Photobucket. Do you have any advice?