Adsorption of amphiphilic peptides to the headgroup region of a lipid bilayer is a common mode of protein-membrane interactions. Previous studies have shown that adsorption causes membrane thinning. The degree of the thinning depends on the degree of the lateral expansion caused by the peptide adsorption. If this simple molecular mechanism is correct, the degree of lateral expansion and consequently the membrane thinning should depend on the size of the headgroup relative to the cross section of the hydrocarbon chains. Previously we have established the connection between the alamethicin insertion transition and the membrane thinning effect. In this paper we use oriented circular dichroism to study the effect of varying the size of the headgroup, while maintaining a constant cross section of the lipid chains, on the insertion transition. A simple quantitative prediction agrees very well with the experiment.