Advice on sand sifter stars

[Raideress]

I had the small fat sand sifter and got another as he is fat, but now wondering is he fat or just different to the thin one. Fat one never climbs thin one just climbed back wall, maybe first one is too fat to climb? I do over feed, in a 4 foot tank 300 litre

 

[Raideress]

Pretty sure fat one eats the pellet food too, have massive amphipod colony too