You have to get 2 binary images. Each one containing only the circles of less than, or greater than 100 pixels. See my Image Segmentation Tutorial where I extract out coins of different diameters using find() and ismember(). http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862 Let me know if you can't adapt it. If that is the case, attach your image and your best attempt at adapting the code.
Then call imdilate() on each image.
se1 = strlen('disk', 20)
dilated1 = imdilate(binaryImage1, se1);
se2 = strlen('disk', 10)
dilated2 = imdilate(binaryImage2, se2);
