Before revision: The reason for this is that the lipids in a fluid bilayer rearrange easily so, unlike a bulk material where the resistance to expansion comes from hydrophobic interactions, the resistance to expansion in a bilayer is a result of the extra hydrophobic area exposed to water upon pulling the lipids apart.D.
After revision: The resistance to expansion in a bilayer is a result of the extra hydrophobic area exposed to water upon pulling the lipids apart.D.