Tank size for foxface vs. rabbitfish

[BL1]

I was checking out Live Aquaria today and I was looking at the rabbitfish and foxfaces ... I'm wondering though, they both grow to the same size, why is it recommended that the foxface have a min. 70g tank and the rabbitfish needs a min. 100g tank then?
Drs. Foster and Smith Diver's Den - Siganus uspi

Drs. Foster and Smith Diver's Den - Siganus doliatus

Just wondering because I'm leaving for a few months and when I come back I'm really considering turning my 75g into SW. THANKS!!!

 

[PRC]

I would figure that the rabbit fish need more swimming room. They like to swim for distance, the foxface just kinda' meanders about.

 

[BL1]

Thanks PRC, that sucks ... that is a cool looking rabbitfish

 

[OhNoNemo]

IDK,sometimes i think they just make up any ol' tank size..I mean,who has a 70 or a 100.Most tanks are either 75,90,120,125,150, and on up.Plus wouldn't a 100 and a 70 have close to the same footprint?

 

[yote]

The difference is in the fish itself.They are both rabbit fish.But the Blue-Lined is more active.