First, I would label regions in your image:
bwl = bwlabel(img,8);
Area of regions:
a = regionprops(img,'area');
a = {a.Area}; %turn into cell array
Create a mask for what you want to be dilated:
xarea = 10;
idx = find(cellfun(@(x)x<xarea,a));
bwl2 = sum(reshape(cell2mat(arrayfun(@(x)x==bwl,idx,'un',0)),size(img,1),size(img,2),[]),3);
Dilate the mask:
bwl2 = bwmorph(bwl2,'dilate',1);
And finally, apply this dilated mask back to the original bw image:
img = img | bwl2;
