File column and rmagick not creating resizing some images

I am new to RoR. I am trying to use file_column and rmagick to resize
images uploaded to my RoR app. The problem is that only images larger
than the dimensions specified in my code will resize. For instance, if
the uploaded image is 98x98, then the new images will remain that size.
How can I enlarge images using file_column and rmagick?

Here is the code in my model:

file_column :image_file, :magick => { :versions => {
:thumb => {:size => “100x100>”},
:medium => {:size => “640x480>”}