RBTA grows and shrinks, normal?

[diablo243]

Have a RBTA and it seems to thrive, gets HUGE but randomly it's shrinks down to next to nothing, then a few hours later normal. Is this normal? If so why do they do it? Thanks all

Yesterday

Now

 

[diablo243]

Oh should mention too the thing eats like a horse! And this isn't the first time its shrank like this, it just is all over the place in shape and size.