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I just wanna know, is it normal for 3-4 inch RES' bottom neck becomes big as they breath? or my turtle is sick?

That's normal. They will pretty much do that everytime the surface for air.

Yep thats normal like tammi said

My turtle does that especially when he basks.

You should be worried only if you hear a wheezing sound when they breathe.

my 2" turtle does it, its normal

One of my turtle have this wheezing sound when i pick him up... normal too? only when i pick him up...

Yep its his hiss

Yes, that's normal. Breathing quickly will produce that hissing sound, like when
a turtle is lifted, surprised, etc.

The hissing noise usually happens because when you spook a turtle they pull their limbs into their shell, and that causes them to expel all the air in their lungs very quickly ^^