Your code enlarges every ROI by 10 pixels, but does not store the new ROI in the ROI manager. You are missing the roiManager("Update");
command, which you get when running the macro recorder while clicking the Update button in the ROI Manager window.
counts=roiManager("count");
for(i=0; i<counts; i++) {
roiManager("Select", i);
run("Enlarge...", "enlarge=10");
roiManager("Update");
}