What you need for a gasket depends a lot on the thermal expansion characteristics of the mating materials. The more difference in the expansion (and the bigger the piece), the more motion the parts are going to see. Since most gasket materials have limited shear capability, the bigger the motion the thicker the gasket has to be to limit the shear stress. ie: sealing fiberglass to fiberglass is easy - they move the same and a thin gasket is OK. Sealing aluminum to fiberglass gets tricky as the two pieces move differently, and the bigger the pieces, the thicker the gasket has to be so it doesn't fail in shear.