Instanton size modulus ($\rho >0$)

The scale $\rho >0$ in a BPST instanton changes the width of its action density while leaving total action unchanged. This reflects conformal invariance of four-dimensional classical Yang-Mills action. As $\rho \to 0$, gauge curvature concentrates at the center and the smooth family approaches a bubbling boundary, not another smooth zero-size instanton.