If i understand your question correctly you want to detect the overlapping shapes in your given image. 'regionprops' can be applied to look for shapes with higher solidity. Also, You can try to use ellipse measurements for detecting the smaller blobs.
props = regionprops(mask, 'Solidity', 'MajorAxisLength', 'MinorAxisLength');
For circles the minor and major axes'lengths should be roughly equal.
The following blogs posts might be a good starting point:
