If by "tighter" you mean 4032 and "looser" 2618, then you need to know which Icon piston series you're looking into. IIRC, the FHR series are 4032, and the others -- including all the BBM stroker pistons I've seen of theirs -- are 2618.

Last time I checked, Ross only used 2618; Diamond used both materials, depending upon the application; JE SRPs were 4032, but the "regular" JEs were 2618; RaceTec is 2618 and AutoTec is 4032; Mahle uses both, depending upon the application... etc.

Ya gotta do your research. scope

Oh, you can put me in the "Icon pistons are 'OK', but there are others I like better" category. Their big advantage is that they stock a wide variety of BBM bore & stroke options that most other piston companies don't.